| Doc. no. | N2806=08-0316 |
| Date: | 2008-12-07 |
| Project: | Programming Language C++ |
| Reply to: | Howard Hinnant <howard.hinnant@gmail.com> |
Reference ISO/IEC IS 14882:2003(E)
Also see:
The purpose of this document is to record the status of issues which have come before the Library Working Group (LWG) of the ANSI (J16) and ISO (WG21) C++ Standards Committee. Issues represent potential defects in the ISO/IEC IS 14882:2003(E) document.
This document contains only library issues which are actively being considered by the Library Working Group. That is, issues which have a status of New, Open, Ready, and Review. See Library Defect Reports List for issues considered defects and Library Closed Issues List for issues considered closed.
The issues in these lists are not necessarily formal ISO Defect Reports (DR's). While some issues will eventually be elevated to official Defect Report status, other issues will be disposed of in other ways. See Issue Status.
Prior to Revision 14, library issues lists existed in two slightly different versions; a Committee Version and a Public Version. Beginning with Revision 14 the two versions were combined into a single version.
This document includes [bracketed italicized notes] as a reminder to the LWG of current progress on issues. Such notes are strictly unofficial and should be read with caution as they may be incomplete or incorrect. Be aware that LWG support for a particular resolution can quickly change if new viewpoints or killer examples are presented in subsequent discussions.
For the most current official version of this document see http://www.open-std.org/jtc1/sc22/wg21/. Requests for further information about this document should include the document number above, reference ISO/IEC 14882:2003(E), and be submitted to Information Technology Industry Council (ITI), 1250 Eye Street NW, Washington, DC 20005.
Public information as to how to obtain a copy of the C++ Standard, join the standards committee, submit an issue, or comment on an issue can be found in the comp.std.c++ FAQ.
New - The issue has not yet been reviewed by the LWG. Any Proposed Resolution is purely a suggestion from the issue submitter, and should not be construed as the view of LWG.
Open - The LWG has discussed the issue but is not yet ready to move the issue forward. There are several possible reasons for open status:
A Proposed Resolution for an open issue is still not be construed as the view of LWG. Comments on the current state of discussions are often given at the end of open issues in an italic font. Such comments are for information only and should not be given undue importance.
Dup - The LWG has reached consensus that the issue is a duplicate of another issue, and will not be further dealt with. A Rationale identifies the duplicated issue's issue number.
NAD - The LWG has reached consensus that the issue is not a defect in the Standard.
NAD Editorial - The LWG has reached consensus that the issue can either be handled editorially, or is handled by a paper (usually linked to in the rationale).
NAD Future - In addition to the regular status, the LWG believes that this issue should be revisited at the next revision of the standard.
Review - Exact wording of a Proposed Resolution is now available for review on an issue for which the LWG previously reached informal consensus.
Tentatively Ready - The issue has been reviewed online, but not in a meeting, and some support has been formed for the proposed resolution. Tentatively Ready issues may be moved to Ready and forwarded to full committee within the same meeting. Unlike Ready issues they will be reviewed in subcommittee prior to forwarding to full committee.
Ready - The LWG has reached consensus that the issue is a defect in the Standard, the Proposed Resolution is correct, and the issue is ready to forward to the full committee for further action as a Defect Report (DR).
DR - (Defect Report) - The full J16 committee has voted to forward the issue to the Project Editor to be processed as a Potential Defect Report. The Project Editor reviews the issue, and then forwards it to the WG21 Convenor, who returns it to the full committee for final disposition. This issues list accords the status of DR to all these Defect Reports regardless of where they are in that process.
TC1 - (Technical Corrigenda 1) - The full WG21 committee has voted to accept the Defect Report's Proposed Resolution as a Technical Corrigenda. Action on this issue is thus complete and no further action is possible under ISO rules.
CD1 - (Committee Draft 2008) - The full WG21 committee has voted to accept the Defect Report's Proposed Resolution into the Fall 2008 Committee Draft.
TRDec - (Decimal TR defect) - The LWG has voted to accept the Defect Report's Proposed Resolution into the Decimal TR. Action on this issue is thus complete and no further action is expected.
WP - (Working Paper) - The proposed resolution has not been accepted as a Technical Corrigendum, but the full WG21 committee has voted to apply the Defect Report's Proposed Resolution to the working paper.
Pending - This is a status qualifier. When prepended to a status this indicates the issue has been processed by the committee, and a decision has been made to move the issue to the associated unqualified status. However for logistical reasons the indicated outcome of the issue has not yet appeared in the latest working paper.
Issues are always given the status of New when they first appear on the issues list. They may progress to Open or Review while the LWG is actively working on them. When the LWG has reached consensus on the disposition of an issue, the status will then change to Dup, NAD, or Ready as appropriate. Once the full J16 committee votes to forward Ready issues to the Project Editor, they are given the status of Defect Report ( DR). These in turn may become the basis for Technical Corrigenda (TC), or are closed without action other than a Record of Response (RR ). The intent of this LWG process is that only issues which are truly defects in the Standard move to the formal ISO DR status.
Section: 23.2.6 [vector] Status: Open Submitter: AFNOR Date: 1998-10-07
View all other issues in [vector].
View all issues with Open status.
Discussion:
vector<bool> is not a container as its reference and pointer types are not references and pointers.
Also it forces everyone to have a space optimization instead of a speed one.
See also: 99-0008 == N1185 Vector<bool> is Nonconforming, Forces Optimization Choice.
[In Santa Cruz the LWG felt that this was Not A Defect.]
[In Dublin many present felt that failure to meet Container requirements was a defect. There was disagreement as to whether or not the optimization requirements constituted a defect.]
[The LWG looked at the following resolutions in some detail:
* Not A Defect.
* Add a note explaining that vector<bool> does not meet
Container requirements.
* Remove vector<bool>.
* Add a new category of container requirements which
vector<bool> would meet.
* Rename vector<bool>.
No alternative had strong, wide-spread, support and every alternative
had at least one "over my dead body" response.
There was also mention of a transition scheme something like (1) add
vector_bool and deprecate vector<bool> in the next standard. (2)
Remove vector<bool> in the following standard.]
[Modifying container requirements to permit returning proxies (thus allowing container requirements conforming vector<bool>) was also discussed.]
[It was also noted that there is a partial but ugly workaround in that vector<bool> may be further specialized with a customer allocator.]
[Kona: Herb Sutter presented his paper J16/99-0035==WG21/N1211, vector<bool>: More Problems, Better Solutions. Much discussion of a two step approach: a) deprecate, b) provide replacement under a new name. LWG straw vote on that: 1-favor, 11-could live with, 2-over my dead body. This resolution was mentioned in the LWG report to the full committee, where several additional committee members indicated over-my-dead-body positions.]
Discussed at Lillehammer. General agreement that we should deprecate vector<bool> and introduce this functionality under a different name, e.g. bit_vector. This might make it possible to remove the vector<bool> specialization in the standard that comes after C++0x. There was also a suggestion that in C++0x we could additional say that it's implementation defined whether vector<bool> refers to the specialization or to the primary template, but there wasn't general agreement that this was a good idea.
We need a paper for the new bit_vector class.
Proposed resolution:
[ Batavia: The LWG feels we need something closer to SGI's bitvector to ease migration from vector<bool>. Although some of the funcitonality from N2050 could well be used in such a template. The concern is easing the API migration for those users who want to continue using a bit-packed container. Alan and Beman to work. ]
Section: 27.7 [string.streams], 27.8 [file.streams] Status: Open Submitter: Angelika Langer Date: 1999-02-22
View all other issues in [string.streams].
View all issues with Open status.
Discussion:
The following question came from Thorsten Herlemann:
You can set a mode when constructing or opening a file-stream or filebuf, e.g. ios::in, ios::out, ios::binary, ... But how can I get that mode later on, e.g. in my own operator << or operator >> or when I want to check whether a file-stream or file-buffer object passed as parameter is opened for input or output or binary? Is there no possibility? Is this a design-error in the standard C++ library?
It is indeed impossible to find out what a stream's or stream buffer's open mode is, and without that knowledge you don't know how certain operations behave. Just think of the append mode.
Both streams and stream buffers should have a mode() function that returns the current open mode setting.
[ post Bellevue: Alisdair requested to re-Open. ]
Proposed resolution:
For stream buffers, add a function to the base class as a non-virtual function qualified as const to 27.5.2 [streambuf]:
openmode mode() const;
Returns the current open mode.
With streams, I'm not sure what to suggest. In principle, the mode could already be returned by ios_base, but the mode is only initialized for file and string stream objects, unless I'm overlooking anything. For this reason it should be added to the most derived stream classes. Alternatively, it could be added to basic_ios and would be default initialized in basic_ios<>::init().
Rationale:
This might be an interesting extension for some future, but it is not a defect in the current standard. The Proposed Resolution is retained for future reference.
Section: 25.3.7 [alg.min.max] Status: Open Submitter: Mark Rintoul Date: 1999-08-26
View other active issues in [alg.min.max].
View all other issues in [alg.min.max].
View all issues with Open status.
Discussion:
Both std::min and std::max are defined as template functions. This
is very different than the definition of std::plus (and similar
structs) which are defined as function objects which inherit
std::binary_function.
This lack of inheritance leaves std::min and std::max somewhat useless in standard library algorithms which require
a function object that inherits std::binary_function.
[ post Bellevue: Alisdair requested to re-Open. ]
Rationale:
Although perhaps an unfortunate design decision, the omission is not a defect in the current standard. A future standard may wish to consider additional function objects.
Section: 27.5.2 [streambuf] Status: Open Submitter: Martin Sebor Date: 2000-08-12
View all other issues in [streambuf].
View all issues with Open status.
Discussion:
The basic_streambuf members gbump() and pbump() are specified to take an int argument. This requirement prevents the functions from effectively manipulating buffers larger than std::numeric_limits<int>::max() characters. It also makes the common use case for these functions somewhat difficult as many compilers will issue a warning when an argument of type larger than int (such as ptrdiff_t on LLP64 architectures) is passed to either of the function. Since it's often the result of the subtraction of two pointers that is passed to the functions, a cast is necessary to silence such warnings. Finally, the usage of a native type in the functions signatures is inconsistent with other member functions (such as sgetn() and sputn()) that manipulate the underlying character buffer. Those functions take a streamsize argument.
Proposed resolution:
Change the signatures of these functions in the synopsis of template class basic_streambuf (27.5.2) and in their descriptions (27.5.2.3.1, p4 and 27.5.2.3.2, p4) to take a streamsize argument.
Although this change has the potential of changing the ABI of the library, the change will affect only platforms where int is different than the definition of streamsize. However, since both functions are typically inline (they are on all known implementations), even on such platforms the change will not affect any user code unless it explicitly relies on the existing type of the functions (e.g., by taking their address). Such a possibility is IMO quite remote.
Alternate Suggestion from Howard Hinnant, c++std-lib-7780:
This is something of a nit, but I'm wondering if streamoff wouldn't be a better choice than streamsize. The argument to pbump and gbump MUST be signed. But the standard has this to say about streamsize (27.4.1/2/Footnote):
[Footnote: streamsize is used in most places where ISO C would use size_t. Most of the uses of streamsize could use size_t, except for the strstreambuf constructors, which require negative values. It should probably be the signed type corresponding to size_t (which is what Posix.2 calls ssize_t). --- end footnote]
This seems a little weak for the argument to pbump and gbump. Should we ever really get rid of strstream, this footnote might go with it, along with the reason to make streamsize signed.
Rationale:
The LWG believes this change is too big for now. We may wish to reconsider this for a future revision of the standard. One possibility is overloading pbump, rather than changing the signature.
[ [2006-05-04: Reopened at the request of Chris (Krzysztof ?elechowski)] ]
Section: 25.1.4 [alg.foreach] Status: Open Submitter: Angelika Langer Date: 2001-01-03
View all other issues in [alg.foreach].
View all issues with Open status.
Discussion:
The specification of the for_each algorithm does not have a "Requires" section, which means that there are no restrictions imposed on the function object whatsoever. In essence it means that I can provide any function object with arbitrary side effects and I can still expect a predictable result. In particular I can expect that the function object is applied exactly last - first times, which is promised in the "Complexity" section.
I don't see how any implementation can give such a guarantee without imposing requirements on the function object.
Just as an example: consider a function object that removes elements from the input sequence. In that case, what does the complexity guarantee (applies f exactly last - first times) mean?
One can argue that this is obviously a nonsensical application and a theoretical case, which unfortunately it isn't. I have seen programmers shooting themselves in the foot this way, and they did not understand that there are restrictions even if the description of the algorithm does not say so.
[Lillehammer: This is more general than for_each. We don't want the function object in transform invalidiating iterators either. There should be a note somewhere in clause 17 (17, not 25) saying that user code operating on a range may not invalidate iterators unless otherwise specified. Bill will provide wording.]
Proposed resolution:
Section: 27.6 [iostream.format] Status: Open Submitter: Martin Sebor Date: 2001-03-19
View all other issues in [iostream.format].
View all issues with Open status.
Discussion:
The descriptions of the constructors of basic_istream<>::sentry (27.6.1.1.3 [istream::sentry]) and basic_ostream<>::sentry (27.6.2.4 [ostream::sentry]) do not explain what the functions do in case an exception is thrown while they execute. Some current implementations allow all exceptions to propagate, others catch them and set ios_base::badbit instead, still others catch some but let others propagate.
The text also mentions that the functions may call setstate(failbit) (without actually saying on what object, but presumably the stream argument is meant). That may have been fine for basic_istream<>::sentry prior to issue 195, since the function performs an input operation which may fail. However, issue 195 amends 27.6.1.1.3 [istream::sentry], p2 to clarify that the function should actually call setstate(failbit | eofbit), so the sentence in p3 is redundant or even somewhat contradictory.
The same sentence that appears in 27.6.2.4 [ostream::sentry], p3 doesn't seem to be very meaningful for basic_istream<>::sentry which performs no input. It is actually rather misleading since it would appear to guide library implementers to calling setstate(failbit) when os.tie()->flush(), the only called function, throws an exception (typically, it's badbit that's set in response to such an event).
Additional comments from Martin, who isn't comfortable with the current proposed resolution (see c++std-lib-11530)
The istream::sentry ctor says nothing about how the function deals with exemptions (27.6.1.1.2, p1 says that the class is responsible for doing "exception safe"(*) prefix and suffix operations but it doesn't explain what level of exception safety the class promises to provide). The mockup example of a "typical implementation of the sentry ctor" given in 27.6.1.1.2, p6, removed in ISO/IEC 14882:2003, doesn't show exception handling, either. Since the ctor is not classified as a formatted or unformatted input function, the text in 27.6.1.1, p1 through p4 does not apply. All this would seem to suggest that the sentry ctor should not catch or in any way handle exceptions thrown from any functions it may call. Thus, the typical implementation of an istream extractor may look something like [1].
The problem with [1] is that while it correctly sets ios::badbit if an exception is thrown from one of the functions called from the sentry ctor, if the sentry ctor reaches EOF while extracting whitespace from a stream that has eofbit or failbit set in exceptions(), it will cause an ios::failure to be thrown, which will in turn cause the extractor to set ios::badbit.
The only straightforward way to prevent this behavior is to move the definition of the sentry object in the extractor above the try block (as suggested by the example in 22.2.8, p9 and also indirectly supported by 27.6.1.3, p1). See [2]. But such an implementation will allow exceptions thrown from functions called from the ctor to freely propagate to the caller regardless of the setting of ios::badbit in the stream object's exceptions().
So since neither [1] nor [2] behaves as expected, the only possible solution is to have the sentry ctor catch exceptions thrown from called functions, set badbit, and propagate those exceptions if badbit is also set in exceptions(). (Another solution exists that deals with both kinds of sentries, but the code is non-obvious and cumbersome -- see [3].)
Please note that, as the issue points out, current libraries do not behave consistently, suggesting that implementors are not quite clear on the exception handling in istream::sentry, despite the fact that some LWG members might feel otherwise. (As documented by the parenthetical comment here: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1480.html#309)
Also please note that those LWG members who in Copenhagen felt that "a sentry's constructor should not catch exceptions, because sentries should only be used within (un)formatted input functions and that exception handling is the responsibility of those functions, not of the sentries," as noted here http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2001/n1310.html#309 would in effect be either arguing for the behavior described in [1] or for extractors implemented along the lines of [3].
The original proposed resolution (Revision 25 of the issues list) clarifies the role of the sentry ctor WRT exception handling by making it clear that extractors (both library or user-defined) should be implemented along the lines of [2] (as opposed to [1]) and that no exception thrown from the callees should propagate out of either function unless badbit is also set in exceptions().
[1] Extractor that catches exceptions thrown from sentry:
struct S { long i; };
istream& operator>> (istream &strm, S &s)
{
ios::iostate err = ios::goodbit;
try {
const istream::sentry guard (strm, false);
if (guard) {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
}
catch (...) {
bool rethrow;
try {
strm.setstate (ios::badbit);
rethrow = false;
}
catch (...) {
rethrow = true;
}
if (rethrow)
throw;
}
if (err)
strm.setstate (err);
return strm;
}
[2] Extractor that propagates exceptions thrown from sentry:
istream& operator>> (istream &strm, S &s)
{
istream::sentry guard (strm, false);
if (guard) {
ios::iostate err = ios::goodbit;
try {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
catch (...) {
bool rethrow;
try {
strm.setstate (ios::badbit);
rethrow = false;
}
catch (...) {
rethrow = true;
}
if (rethrow)
throw;
}
if (err)
strm.setstate (err);
}
return strm;
}
[3] Extractor that catches exceptions thrown from sentry but doesn't set badbit if the exception was thrown as a result of a call to strm.clear().
istream& operator>> (istream &strm, S &s)
{
const ios::iostate state = strm.rdstate ();
const ios::iostate except = strm.exceptions ();
ios::iostate err = std::ios::goodbit;
bool thrown = true;
try {
const istream::sentry guard (strm, false);
thrown = false;
if (guard) {
use_facet<num_get<char> >(strm.getloc ())
.get (istreambuf_iterator<char>(strm),
istreambuf_iterator<char>(),
strm, err, s.i);
}
}
catch (...) {
if (thrown && state & except)
throw;
try {
strm.setstate (ios::badbit);
thrown = false;
}
catch (...) {
thrown = true;
}
if (thrown)
throw;
}
if (err)
strm.setstate (err);
return strm;
}
[Pre-Berlin] Reopened at the request of Paolo Carlini and Steve Clamage.
[Pre-Portland] A relevant newsgroup post:
The current proposed resolution of issue #309 (http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#309) is unacceptable. I write commerical software and coding around this makes my code ugly, non-intuitive, and requires comments referring people to this very issue. Following is the full explanation of my experience.
In the course of writing software for commercial use, I constructed std::ifstream's based on user-supplied pathnames on typical POSIX systems.
It was expected that some files that opened successfully might not read successfully -- such as a pathname which actually refered to a directory. Intuitively, I expected the streambuffer underflow() code to throw an exception in this situation, and recent implementations of libstdc++'s basic_filebuf do just that (as well as many of my own custom streambufs).
I also intuitively expected that the istream code would convert these exceptions to the "badbit' set on the stream object, because I had not requested exceptions. I refer to 27.6.1.1. P4.
However, this was not the case on at least two implementations -- if the first thing I did with an istream was call operator>>( T& ) for T among the basic arithmetic types and std::string. Looking further I found that the sentry's constructor was invoking the exception when it pre-scanned for whitespace, and the extractor function (operator>>()) was not catching exceptions in this situation.
So, I was in a situation where setting 'noskipws' would change the istream's behavior even though no characters (whitespace or not) could ever be successfully read.
Also, calling .peek() on the istream before calling the extractor() changed the behavior (.peek() had the effect of setting the badbit ahead of time).
I found this all to be so inconsistent and inconvenient for me and my code design, that I filed a bugzilla entry for libstdc++. I was then told that the bug cannot be fixed until issue #309 is resolved by the committee.
Proposed resolution:
Rationale:
The LWG agrees there is minor variation between implementations, but believes that it doesn't matter. This is a rarely used corner case. There is no evidence that this has any commercial importance or that it causes actual portability problems for customers trying to write code that runs on multiple implementations.
Section: 27.6.1.3 [istream.unformatted] Status: Open Submitter: Howard Hinnant Date: 2001-10-09
View all other issues in [istream.unformatted].
View all issues with Open status.
Discussion:
I think we have a defect.
According to lwg issue 60 which is now a dr, the description of seekg in 27.6.1.3 [istream.unformatted] paragraph 38 now looks like:
Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail() != true, executes rdbuf()->pubseekpos( pos).
And according to lwg issue 243 which is also now a dr, 27.6.1.3, paragraph 1 looks like:
Each unformatted input function begins execution by constructing an object of class sentry with the default argument noskipws (second) argument true. If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input. Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (using charT()) in the first location of the array. If an exception is thrown during input then ios::badbit is turned on in *this'ss error state. If (exception()&badbit)!= 0 then the exception is rethrown. It also counts the number of characters extracted. If no exception has been thrown it ends by storing the count in a member object and returning the value specified. In any event the sentry object is destroyed before leaving the unformatted input function.
And finally 27.6.1.1.2/5 says this about sentry:
If, after any preparation is completed, is.good() is true, ok_ != false otherwise, ok_ == false.
So although the seekg paragraph says that the operation proceeds if !fail(), the behavior of unformatted functions says the operation proceeds only if good(). The two statements are contradictory when only eofbit is set. I don't think the current text is clear which condition should be respected.
Further discussion from Redmond:
PJP: It doesn't seem quite right to say that seekg is "unformatted". That makes specific claims about sentry that aren't quite appropriate for seeking, which has less fragile failure modes than actual input. If we do really mean that it's unformatted input, it should behave the same way as other unformatted input. On the other hand, "principle of least surprise" is that seeking from EOF ought to be OK.
Pre-Berlin: Paolo points out several problems with the proposed resolution in Ready state:
Proposed resolution:
Change 27.6.1.3 [istream.unformatted] to:
Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted, does not affect the value returned by subsequent calls to gcount(), and does not examine the value returned by the sentry object. After constructing a sentry object, if fail() != true, executes rdbuf()->pubseekpos(pos). In case of success, the function calls clear(). In case of failure, the function calls setstate(failbit) (which may throw ios_base::failure).
[Lillehammer: Matt provided wording.]
Rationale:
In C, fseek does clear EOF. This is probably what most users would expect. We agree that having eofbit set should not deter a seek, and that a successful seek should clear eofbit. Note that fail() is true only if failbit or badbit is set, so using !fail(), rather than good(), satisfies this goal.
Section: 21 [strings], 23 [containers], 27 [input.output] Status: Open Submitter: Martin Sebor Date: 2001-10-09
View all other issues in [strings].
View all issues with Open status.
Discussion:
The synopses of the C++ library headers clearly show which names are required to be defined in each header. Since in order to implement the classes and templates defined in these headers declarations of other templates (but not necessarily their definitions) are typically necessary the standard in 17.4.4, p1 permits library implementers to include any headers needed to implement the definitions in each header.
For instance, although it is not explicitly specified in the synopsis of <string>, at the point of definition of the std::basic_string template the declaration of the std::allocator template must be in scope. All current implementations simply include <memory> from within <string>, either directly or indirectly, to bring the declaration of std::allocator into scope.
Additionally, however, some implementation also include <istream> and <ostream> at the top of <string> to bring the declarations of std::basic_istream and std::basic_ostream into scope (which are needed in order to implement the string inserter and extractor operators (21.3.7.9 [lib.string.io])). Other implementations only include <iosfwd>, since strictly speaking, only the declarations and not the full definitions are necessary.
Obviously, it is possible to implement <string> without actually providing the full definitions of all the templates std::basic_string uses (std::allocator, std::basic_istream, and std::basic_ostream). Furthermore, not only is it possible, doing so is likely to have a positive effect on compile-time efficiency.
But while it may seem perfectly reasonable to expect a program that uses the std::basic_string insertion and extraction operators to also explicitly include <istream> or <ostream>, respectively, it doesn't seem reasonable to also expect it to explicitly include <memory>. Since what's reasonable and what isn't is highly subjective one would expect the standard to specify what can and what cannot be assumed. Unfortunately, that isn't the case.
The examples below demonstrate the issue.
Example 1:
It is not clear whether the following program is complete:
#include <string>
extern std::basic_ostream<char> &strm;
int main () {
strm << std::string ("Hello, World!\n");
}
or whether one must explicitly include <memory> or <ostream> (or both) in addition to <string> in order for the program to compile.
Example 2:
Similarly, it is unclear whether the following program is complete:
#include <istream>
extern std::basic_iostream<char> &strm;
int main () {
strm << "Hello, World!\n";
}
or whether one needs to explicitly include <ostream>, and perhaps even other headers containing the definitions of other required templates:
#include <ios>
#include <istream>
#include <ostream>
#include <streambuf>
extern std::basic_iostream<char> &strm;
int main () {
strm << "Hello, World!\n";
}
Example 3:
Likewise, it seems unclear whether the program below is complete:
#include <iterator>
bool foo (std::istream_iterator<int> a, std::istream_iterator<int> b)
{
return a == b;
}
int main () { }
or whether one should be required to include <istream>.
There are many more examples that demonstrate this lack of a requirement. I believe that in a good number of cases it would be unreasonable to require that a program explicitly include all the headers necessary for a particular template to be specialized, but I think that there are cases such as some of those above where it would be desirable to allow implementations to include only as much as necessary and not more.
[ post Bellevue: ]
Position taken in prior reviews is that the idea of a table of header dependencies is a good one. Our view is that a full paper is needed to do justice to this, and we've made that recommendation to the issue author.
Proposed resolution:
For every C++ library header, supply a minimum set of other C++ library headers that are required to be included by that header. The proposed list is below (C++ headers for C Library Facilities, table 12 in 17.4.1.2, p3, are omitted):
+------------+--------------------+ | C++ header |required to include | +============+====================+ |<algorithm> | | +------------+--------------------+ |<bitset> | | +------------+--------------------+ |<complex> | | +------------+--------------------+ |<deque> |<memory> | +------------+--------------------+ |<exception> | | +------------+--------------------+ |<fstream> |<ios> | +------------+--------------------+ |<functional>| | +------------+--------------------+ |<iomanip> |<ios> | +------------+--------------------+ |<ios> |<streambuf> | +------------+--------------------+ |<iosfwd> | | +------------+--------------------+ |<iostream> |<istream>, <ostream>| +------------+--------------------+ |<istream> |<ios> | +------------+--------------------+ |<iterator> | | +------------+--------------------+ |<limits> | | +------------+--------------------+ |<list> |<memory> | +------------+--------------------+ |<locale> | | +------------+--------------------+ |<map> |<memory> | +------------+--------------------+ |<memory> | | +------------+--------------------+ |<new> |<exception> | +------------+--------------------+ |<numeric> | | +------------+--------------------+ |<ostream> |<ios> | +------------+--------------------+ |<queue> |<deque> | +------------+--------------------+ |<set> |<memory> | +------------+--------------------+ |<sstream> |<ios>, <string> | +------------+--------------------+ |<stack> |<deque> | +------------+--------------------+ |<stdexcept> | | +------------+--------------------+ |<streambuf> |<ios> | +------------+--------------------+ |<string> |<memory> | +------------+--------------------+ |<strstream> | | +------------+--------------------+ |<typeinfo> |<exception> | +------------+--------------------+ |<utility> | | +------------+--------------------+ |<valarray> | | +------------+--------------------+ |<vector> |<memory> | +------------+--------------------+
Rationale:
The portability problem is real. A program that works correctly on one implementation might fail on another, because of different header dependencies. This problem was understood before the standard was completed, and it was a conscious design choice.
One possible way to deal with this, as a library extension, would be an <all> header.
Hinnant: It's time we dealt with this issue for C++0X. Reopened.
Section: 22.2.1.4 [locale.codecvt] Status: Open Submitter: Martin Sebor Date: 2002-08-30
View all other issues in [locale.codecvt].
View all issues with Open status.
Discussion:
It seems that the descriptions of codecvt do_in() and do_out() leave sufficient room for interpretation so that two implementations of codecvt may not work correctly with the same filebuf. Specifically, the following seems less than adequately specified:
Finally, the conditions described at the end of 22.2.1.4.2 [locale.codecvt.virtuals], p4 don't seem to be possible:
"A return value of partial, if (from_next == from_end), indicates that either the destination sequence has not absorbed all the available destination elements, or that additional source elements are needed before another destination element can be produced."
If the value is partial, it's not clear to me that (from_next ==from_end) could ever hold if there isn't enough room in the destination buffer. In order for (from_next==from_end) to hold, all characters in that range must have been successfully converted (according to 22.2.1.4.2 [locale.codecvt.virtuals], p2) and since there are no further source characters to convert, no more room in the destination buffer can be needed.
It's also not clear to me that (from_next==from_end) could ever hold if additional source elements are needed to produce another destination character (not element as incorrectly stated in the text). partial is returned if "not all source characters have been converted" according to Table 53, which also implies that (from_next==from) does NOT hold.
Could it be that the intended qualifying condition was actually (from_next != from_end), i.e., that the sentence was supposed to read
"A return value of partial, if (from_next != from_end),..."
which would make perfect sense, since, as far as I understand it, partial can only occur if (from_next != from_end)?
[Lillehammer: Defer for the moment, but this really needs to be fixed. Right now, the description of codecvt is too vague for it to be a useful contract between providers and clients of codecvt facets. (Note that both vendors and users can be both providers and clients of codecvt facets.) The major philosophical issue is whether the standard should only describe mappings that take a single wide character to multiple narrow characters (and vice versa), or whether it should describe fully general N-to-M conversions. When the original standard was written only the former was contemplated, but today, in light of the popularity of utf8 and utf16, that doesn't seem sufficient for C++0x. Bill supports general N-to-M conversions; we need to make sure Martin and Howard agree.]
Proposed resolution:
Section: 27.6.2.6.1 [ostream.formatted.reqmts] Status: Open Submitter: Martin Sebor Date: 2002-12-27
View all issues with Open status.
Discussion:
There is a contradiction in Formatted output about what bit is supposed to be set if the formatting fails. On sentence says it's badbit and another that it's failbit.
27.6.2.5.1, p1 says in the Common Requirements on Formatted output functions:
... If the generation fails, then the formatted output function
does setstate(ios::failbit), which might throw an exception.
27.6.2.5.2, p1 goes on to say this about Arithmetic Inserters:
... The formatting conversion occurs as if it performed the following code fragment:
bool failed =
use_facet<num_put<charT,ostreambuf_iterator<charT,traits>
> >
(getloc()).put(*this, *this, fill(), val). failed();
... If failed is true then does setstate(badbit) ...
The original intent of the text, according to Jerry Schwarz (see c++std-lib-10500), is captured in the following paragraph:
In general "badbit" should mean that the stream is unusable because of some underlying failure, such as disk full or socket closure; "failbit" should mean that the requested formatting wasn't possible because of some inconsistency such as negative widths. So typically if you clear badbit and try to output something else you'll fail again, but if you clear failbit and try to output something else you'll succeed.
In the case of the arithmetic inserters, since num_put cannot report failure by any means other than exceptions (in response to which the stream must set badbit, which prevents the kind of recoverable error reporting mentioned above), the only other detectable failure is if the iterator returned from num_put returns true from failed().
Since that can only happen (at least with the required iostream specializations) under such conditions as the underlying failure referred to above (e.g., disk full), setting badbit would seem to be the appropriate response (indeed, it is required in 27.6.2.5.2, p1). It follows that failbit can never be directly set by the arithmetic (it can only be set by the sentry object under some unspecified conditions).
The situation is different for other formatted output functions which can fail as a result of the streambuf functions failing (they may do so by means other than exceptions), and which are then required to set failbit.
The contradiction, then, is that ostream::operator<<(int) will set badbit if the disk is full, while operator<<(ostream&, char) will set failbit under the same conditions. To make the behavior consistent, the Common requirements sections for the Formatted output functions should be changed as proposed below.
[Kona: There's agreement that this is a real issue. What we decided at Kona: 1. An error from the buffer (which can be detected either directly from streambuf's member functions or by examining a streambuf_iterator) should always result in badbit getting set. 2. There should never be a circumstance where failbit gets set. That represents a formatting error, and there are no circumstances under which the output facets are specified as signaling a formatting error. (Even more so for string output that for numeric because there's nothing to format.) If we ever decide to make it possible for formatting errors to exist then the facets can signal the error directly, and that should go in clause 22, not clause 27. 3. The phrase "if generation fails" is unclear and should be eliminated. It's not clear whether it's intended to mean a buffer error (e.g. a full disk), a formatting error, or something else. Most people thought it was supposed to refer to buffer errors; if so, we should say so. Martin will provide wording.]
Proposed resolution:
Rationale:
Section: 27.6.2.4 [ostream::sentry] Status: Open Submitter: Martin Sebor Date: 2003-01-05
View other active issues in [ostream::sentry].
View all other issues in [ostream::sentry].
View all issues with Open status.
Discussion:
17.4.4.8, p3 prohibits library dtors from throwing exceptions.
27.6.2.3, p4 says this about the ostream::sentry dtor:
-4- If ((os.flags() & ios_base::unitbuf) && !uncaught_exception())
is true, calls os.flush().
27.6.2.6, p7 that describes ostream::flush() says:
-7- If rdbuf() is not a null pointer, calls rdbuf()->pubsync().
If that function returns ?-1 calls setstate(badbit) (which
may throw ios_base::failure (27.4.4.3)).
That seems like a defect, since both pubsync() and setstate() can throw an exception.
[ The contradiction is real. Clause 17 says destructors may never throw exceptions, and clause 27 specifies a destructor that does throw. In principle we might change either one. We're leaning toward changing clause 17: putting in an "unless otherwise specified" clause, and then putting in a footnote saying the sentry destructor is the only one that can throw. PJP suggests specifying that sentry::~sentry() should internally catch any exceptions it might cause. ]
[ See 418 and 622 for related issues. ]
Proposed resolution:
Section: 27.6.2.4 [ostream::sentry] Status: Open Submitter: Martin Sebor Date: 2003-01-05
View other active issues in [ostream::sentry].
View all other issues in [ostream::sentry].
View all issues with Open status.
Discussion:
While reviewing unformatted input member functions of istream for their behavior when they encounter end-of-file during input I found that the requirements vary, sometimes unexpectedly, and in more than one case even contradict established practice (GNU libstdc++ 3.2, IBM VAC++ 6.0, STLPort 4.5, SunPro 5.3, HP aCC 5.38, Rogue Wave libstd 3.1, and Classic Iostreams).
The following unformatted input member functions set eofbit if they encounter an end-of-file (this is the expected behavior, and also the behavior of all major implementations):
basic_istream<charT, traits>&
get (char_type*, streamsize, char_type);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
get (char_type*, streamsize);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
getline (char_type*, streamsize, char_type);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
getline (char_type*, streamsize);
Also sets failbit if it fails to extract any characters.
basic_istream<charT, traits>&
ignore (int, int_type);
basic_istream<charT, traits>&
read (char_type*, streamsize);
Also sets failbit if it encounters end-of-file.
streamsize readsome (char_type*, streamsize);
The following unformated input member functions set failbit but not eofbit if they encounter an end-of-file (I find this odd since the functions make it impossible to distinguish a general failure from a failure due to end-of-file; the requirement is also in conflict with all major implementation which set both eofbit and failbit):
int_type get();
basic_istream<charT, traits>&
get (char_type&);
These functions only set failbit of they extract no characters, otherwise they don't set any bits, even on failure (I find this inconsistency quite unexpected; the requirement is also in conflict with all major implementations which set eofbit whenever they encounter end-of-file):
basic_istream<charT, traits>&
get (basic_streambuf<charT, traits>&, char_type);
basic_istream<charT, traits>&
get (basic_streambuf<charT, traits>&);
This function sets no bits (all implementations except for STLport and Classic Iostreams set eofbit when they encounter end-of-file):
int_type peek ();
Informally, what we want is a global statement of intent saying that eofbit gets set if we trip across EOF, and then we can take away the specific wording for individual functions. A full review is necessary. The wording currently in the standard is a mishmash, and changing it on an individual basis wouldn't make things better. Dietmar will do this work.
Proposed resolution:
Section: 24.1 [iterator.concepts] Status: Open Submitter: Nathan Myers Date: 2003-06-03
View other active issues in [iterator.concepts].
View all other issues in [iterator.concepts].
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Discussion:
I've been discussing iterator semantics with Dave Abrahams, and a surprise has popped up. I don't think this has been discussed before.
24.1 [iterator.concepts] says that the only operation that can be performed on "singular" iterator values is to assign a non-singular value to them. (It doesn't say they can be destroyed, and that's probably a defect.) Some implementations have taken this to imply that there is no need to initialize the data member of a reverse_iterator<> in the default constructor. As a result, code like
std::vector<std::reverse_iterator<char*> > v(7); v.reserve(1000);
invokes undefined behavior, because it must default-initialize the vector elements, and then copy them to other storage. Of course many other vector operations on these adapters are also left undefined, and which those are is not reliably deducible from the standard.
I don't think that 24.1 was meant to make standard-library iterator types unsafe. Rather, it was meant to restrict what operations may be performed by functions which take general user- and standard iterators as arguments, so that raw pointers would qualify as iterators. However, this is not clear in the text, others have come to the opposite conclusion.
One question is whether the standard iterator adaptors have defined copy semantics. Another is whether they have defined destructor semantics: is
{ std::vector<std::reverse_iterator<char*> > v(7); }
undefined too?
Note this is not a question of whether algorithms are allowed to rely on copy semantics for arbitrary iterators, just whether the types we actually supply support those operations. I believe the resolution must be expressed in terms of the semantics of the adapter's argument type. It should make clear that, e.g., the reverse_iterator<T> constructor is actually required to execute T(), and so copying is defined if the result of T() is copyable.
Issue 235, which defines reverse_iterator's default constructor more precisely, has some relevance to this issue. However, it is not the whole story.
The issue was whether
reverse_iterator() { }
is allowed, vs.
reverse_iterator() : current() { }
The difference is when T is char*, where the first leaves the member uninitialized, and possibly equal to an existing pointer value, or (on some targets) may result in a hardware trap when copied.
8.5 paragraph 5 seems to make clear that the second is required to satisfy DR 235, at least for non-class Iterator argument types.
But that only takes care of reverse_iterator, and doesn't establish a policy for all iterators. (The reverse iterator adapter was just an example.) In particular, does my function
template <typename Iterator>
void f() { std::vector<Iterator> v(7); }
evoke undefined behavior for some conforming iterator definitions? I think it does, now, because vector<> will destroy those singular iterator values, and that's explicitly disallowed.
24.1 shouldn't give blanket permission to copy all singular iterators, because then pointers wouldn't qualify as iterators. However, it should allow copying of that subset of singular iterator values that are default-initialized, and it should explicitly allow destroying any iterator value, singular or not, default-initialized or not.
Related issue: 407
[ We don't want to require all singular iterators to be copyable, because that is not the case for pointers. However, default construction may be a special case. Issue: is it really default construction we want to talk about, or is it something like value initialization? We need to check with core to see whether default constructed pointers are required to be copyable; if not, it would be wrong to impose so strict a requirement for iterators. ]
Proposed resolution:
Section: 22.2.1.1.2 [locale.ctype.virtuals] Status: Open Submitter: Martin Sebor Date: 2003-09-18
View all other issues in [locale.ctype.virtuals].
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Discussion:
The Effects and Returns clauses of the do_widen() member function of the ctype facet fail to specify the behavior of the function on failure. That the function may not be able to simply cast the narrow character argument to the type of the result since doing so may yield the wrong value for some wchar_t encodings. Popular implementations of ctype<wchar_t> that use mbtowc() and UTF-8 as the native encoding (e.g., GNU glibc) will fail when the argument's MSB is set. There is no way for the the rest of locale and iostream to reliably detect this failure.
[Kona: This is a real problem. Widening can fail. It's unclear what the solution should be. Returning WEOF works for the wchar_t specialization, but not in general. One option might be to add a default, like narrow. But that's an incompatible change. Using traits::eof might seem like a good idea, but facets don't have access to traits (a recurring problem). We could have widen throw an exception, but that's a scary option; existing library components aren't written with the assumption that widen can throw.]
Proposed resolution:
Section: 27.4.2.1.6 [ios::Init] Status: Open Submitter: Martin Sebor Date: 2003-09-18
View all issues with Open status.
Discussion:
The dtor of the ios_base::Init object is supposed to call flush() on the 6 standard iostream objects cout, cerr, clog, wcout, wcerr, and wclog. This call may cause an exception to be thrown.
17.4.4.8, p3 prohibits all library destructors from throwing exceptions.
The question is: What should this dtor do if one or more of these calls to flush() ends up throwing an exception? This can happen quite easily if one of the facets installed in the locale imbued in the iostream object throws.
[Kona: We probably can't do much better than what we've got, so the LWG is leaning toward NAD. At the point where the standard stream objects are being cleaned up, the usual error reporting mechanism are all unavailable. And exception from flush at this point will definitely cause problems. A quality implementation might reasonably swallow the exception, or call abort, or do something even more drastic.]
[ See 397 and 622 for related issues. ]
Proposed resolution:
Section: 27.6.1.1.3 [istream::sentry] Status: Open Submitter: Martin Sebor Date: 2003-09-18
View all other issues in [istream::sentry].
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Discussion:
27.6.1.1.3 [istream::sentry], p2 says that istream::sentry ctor prepares for input if is.good() is true. p4 then goes on to say that the ctor sets the sentry::ok_ member to true if the stream state is good after any preparation. 27.6.1.2.1 [istream.formatted.reqmts], p1 then says that a formatted input function endeavors to obtain the requested input if the sentry's operator bool() returns true. Given these requirements, no formatted extractor should ever set failbit if the initial stream rdstate() == eofbit. That is contrary to the behavior of all implementations I tested. The program below prints out eof = 1, fail = 0 eof = 1, fail = 1 on all of them.
#include <sstream>
#include <cstdio>
int main()
{
std::istringstream strm ("1");
int i = 0;
strm >> i;
std::printf ("eof = %d, fail = %d\n",
!!strm.eof (), !!strm.fail ());
strm >> i;
std::printf ("eof = %d, fail = %d\n",
!!strm.eof (), !!strm.fail ());
}
Comments from Jerry Schwarz (c++std-lib-11373):
Jerry Schwarz wrote:
I don't know where (if anywhere) it says it in the standard, but the
formatted extractors are supposed to set failbit if they don't extract
any characters. If they didn't then simple loops like
while (cin >> x);
would loop forever.
Further comments from Martin Sebor:
The question is which part of the extraction should prevent this from happening
by setting failbit when eofbit is already set. It could either be the sentry
object or the extractor. It seems that most implementations have chosen to
set failbit in the sentry [...] so that's the text that will need to be
corrected.
Pre Berlin: This issue is related to 342. If the sentry sets failbit when it finds eofbit already set, then you can never seek away from the end of stream.
Kona: Possibly NAD. If eofbit is set then good() will return false. We then set ok to false. We believe that the sentry's constructor should always set failbit when ok is false, and we also think the standard already says that. Possibly it could be clearer.
Proposed resolution:
Change 27.6.1.1.3 [istream::sentry], p2 to:
explicit sentry(basic_istream<charT,traits>& is , bool noskipws = false);-2- Effects: If is.good() is
truefalse, calls is.setstate(failbit). Otherwise prepares for formatted or unformatted input. ...
Section: 27.5.2.1 [streambuf.cons] Status: Open Submitter: Martin Sebor Date: 2003-09-18
View all other issues in [streambuf.cons].
View all issues with Open status.
Discussion:
The reflector thread starting with c++std-lib-11346 notes that the class template basic_streambuf, along with basic_stringbuf and basic_filebuf, is copy-constructible but that the semantics of the copy constructors are not defined anywhere. Further, different implementations behave differently in this respect: some prevent copy construction of objects of these types by declaring their copy ctors and assignment operators private, others exhibit undefined behavior, while others still give these operations well-defined semantics.
Note that this problem doesn't seem to be isolated to just the three types mentioned above. A number of other types in the library section of the standard provide a compiler-generated copy ctor and assignment operator yet fail to specify their semantics. It's believed that the only types for which this is actually a problem (i.e. types where the compiler-generated default may be inappropriate and may not have been intended) are locale facets. See issue 439.
Proposed resolution:
27.5.2 [lib.streambuf]: Add into the synopsis, public section, just above the destructor declaration:
basic_streambuf(const basic_streambuf& sb); basic_streambuf& operator=(const basic_streambuf& sb);
Insert after 27.5.2.1, paragraph 2:
basic_streambuf(const basic_streambuf& sb);Constructs a copy of sb.
Postcondtions:
eback() == sb.eback() gptr() == sb.gptr() egptr() == sb.egptr() pbase() == sb.pbase() pptr() == sb.pptr() epptr() == sb.epptr() getloc() == sb.getloc()basic_streambuf& operator=(const basic_streambuf& sb);Assigns the data members of sb to this.
Postcondtions:
eback() == sb.eback() gptr() == sb.gptr() egptr() == sb.egptr() pbase() == sb.pbase() pptr() == sb.pptr() epptr() == sb.epptr() getloc() == sb.getloc()Returns: *this.
27.7.1 [lib.stringbuf]:
Option A:
Insert into the basic_stringbuf synopsis in the private section:
basic_stringbuf(const basic_stringbuf&); // not defined basic_stringbuf& operator=(const basic_stringbuf&); // not defined
Option B:
Insert into the basic_stringbuf synopsis in the public section:
basic_stringbuf(const basic_stringbuf& sb); basic_stringbuf& operator=(const basic_stringbuf& sb);27.7.1.1, insert after paragraph 4:
basic_stringbuf(const basic_stringbuf& sb);Constructs an independent copy of sb as if with sb.str(), and with the openmode that sb was constructed with.
Postcondtions:
str() == sb.str() gptr() - eback() == sb.gptr() - sb.eback() egptr() - eback() == sb.egptr() - sb.eback() pptr() - pbase() == sb.pptr() - sb.pbase() getloc() == sb.getloc()Note: The only requirement on epptr() is that it point beyond the initialized range if an output sequence exists. There is no requirement that epptr() - pbase() == sb.epptr() - sb.pbase().
basic_stringbuf& operator=(const basic_stringbuf& sb);After assignment the basic_stringbuf has the same state as if it were initially copy constructed from sb, except that the basic_stringbuf is allowed to retain any excess capacity it might have, which may in turn effect the value of epptr().
27.8.1.1 [lib.filebuf]
Insert at the bottom of the basic_filebuf synopsis:
private: basic_filebuf(const basic_filebuf&); // not defined basic_filebuf& operator=(const basic_filebuf&); // not defined
[Kona: this is an issue for basic_streambuf itself and for its derived classes. We are leaning toward allowing basic_streambuf to be copyable, and specifying its precise semantics. (Probably the obvious: copying the buffer pointers.) We are less sure whether the streambuf derived classes should be copyable. Howard will write up a proposal.]
[Sydney: Dietmar presented a new argument against basic_streambuf being copyable: it can lead to an encapsulation violation. Filebuf inherits from streambuf. Now suppose you inhert a my_hijacking_buf from streambuf. You can copy the streambuf portion of a filebuf to a my_hijacking_buf, giving you access to the pointers into the filebuf's internal buffer. Perhaps not a very strong argument, but it was strong enough to make people nervous. There was weak preference for having streambuf not be copyable. There was weak preference for having stringbuf not be copyable even if streambuf is. Move this issue to open for now. ]
[ 2007-01-12, Howard: Rvalue Reference Recommendations for Chapter 27 recommends protected copy constructor and assignment for basic_streambuf with the same semantics as would be generated by the compiler. These members aid in derived classes implementing move semantics. A protected copy constructor and copy assignment operator do not expose encapsulation more so than it is today as each data member of a basic_streambuf is already both readable and writable by derived classes via various get/set protected member functions (eback(), setp(), etc.). Rather a protected copy constructor and copy assignment operator simply make the job of derived classes implementing move semantics less tedious and error prone. ]
Rationale:
27.5.2 [lib.streambuf]: The proposed basic_streambuf copy constructor and assignment operator are the same as currently implied by the lack of declarations: public and simply copies the data members. This resolution is not a change but a clarification of the current standard.
27.7.1 [lib.stringbuf]: There are two reasonable options: A) Make basic_stringbuf not copyable. This is likely the status-quo of current implementations. B) Reasonable copy semantics of basic_stringbuf can be defined and implemented. A copyable basic_streambuf is arguably more useful than a non-copyable one. This should be considered as new functionality and not the fixing of a defect. If option B is chosen, ramifications from issue 432 are taken into account.
27.8.1.1 [lib.filebuf]: There are no reasonable copy semantics for basic_filebuf.
Section: 27 [input.output] Status: Open Submitter: Martin Sebor Date: 2003-09-18
View all other issues in [input.output].
View all issues with Open status.
Discussion:
A third party test suite tries to exercise istream::ignore(N) with a negative value of N and expects that the implementation will treat N as if it were 0. Our implementation asserts that (N >= 0) holds and aborts the test.
I can't find anything in section 27 that prohibits such values but I don't see what the effects of such calls should be, either (this applies to a number of unformatted input functions as well as some member functions of the basic_streambuf template).
Proposed resolution:
I propose that we add to each function in clause 27 that takes an argument, say N, of type streamsize a Requires clause saying that "N >= 0." The intent is to allow negative streamsize values in calls to precision() and width() but disallow it in calls to streambuf::sgetn(), istream::ignore(), or ostream::write().
[Kona: The LWG agreed that this is probably what we want. However, we need a review to find all places where functions in clause 27 take arguments of type streamsize that shouldn't be allowed to go negative. Martin will do that review.]
Section: 17.5.2.2 [structure.summary] Status: Open Submitter: Martin Sebor Date: 2003-09-18
View all issues with Open status.
Discussion:
The text in 17.3.1.1, p1 says:
"Paragraphs labelled "Note(s):" or "Example(s):" are informative, other
paragraphs are normative."
The library section makes heavy use of paragraphs labeled "Notes(s),"
some of which are clearly intended to be normative (see list 1), while
some others are not (see list 2). There are also those where the intent
is not so clear (see list 3).
List 1 -- Examples of (presumably) normative Notes:
20.8.6.1 [allocator.members], p3,
20.8.6.1 [allocator.members], p10,
21.3.2 [string.cons], p11,
22.1.1.2 [locale.cons], p11,
23.2.2.3 [deque.modifiers], p2,
25.3.7 [alg.min.max], p3,
26.3.6 [complex.ops], p15,
27.5.2.4.3 [streambuf.virt.get], p7.
List 2 -- Examples of (presumably) informative Notes:
18.5.1.3 [new.delete.placement], p3,
21.3.6.6 [string::replace], p14,
22.2.1.4.2 [locale.codecvt.virtuals], p3,
25.1.4 [alg.foreach], p4,
26.3.5 [complex.member.ops], p1,
27.4.2.5 [ios.base.storage], p6.
List 3 -- Examples of Notes that are not clearly either normative
or informative:
22.1.1.2 [locale.cons], p8,
22.1.1.5 [locale.statics], p6,
27.5.2.4.5 [streambuf.virt.put], p4.
None of these lists is meant to be exhaustive.
[Definitely a real problem. The big problem is there's material that doesn't quite fit any of the named paragraph categories (e.g. Effects). Either we need a new kind of named paragraph, or we need to put more material in unnamed paragraphs jsut after the signature. We need to talk to the Project Editor about how to do this. ]
[ Bellevue: Specifics of list 3: First 2 items correct in std (22.1.1.2, 22.1.1.5) Third item should be non-normative (27.5.2.4.5), which Pete will handle editorially. ]
[ post San Francisco: Howard: reopened, needs attention. ]
Proposed resolution:
[Pete: I changed the paragraphs marked "Note" and "Notes" to use "Remark" and "Remarks". Fixed as editorial. This change has been in the WD since the post-Redmond mailing, in 2004. Recommend NAD.]
[ Batavia: We feel that the references in List 2 above should be changed from Remarks to Notes. We also feel that those items in List 3 need to be double checked for the same change. Alan and Pete to review. ]
Section: 22.2.2.1.2 [facet.num.get.virtuals] Status: Open Submitter: Martin Sebor Date: 2003-09-18
View other active issues in [facet.num.get.virtuals].
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Discussion:
The requirements specified in Stage 2 and reiterated in the rationale of DR 221 (and echoed again in DR 303) specify that num_get<charT>:: do_get() compares characters on the stream against the widened elements of "012...abc...ABCX+-"
An implementation is required to allow programs to instantiate the num_get template on any charT that satisfies the requirements on a user-defined character type. These requirements do not include the ability of the character type to be equality comparable (the char_traits template must be used to perform tests for equality). Hence, the num_get template cannot be implemented to support any arbitrary character type. The num_get template must either make the assumption that the character type is equality-comparable (as some popular implementations do), or it may use char_traits<charT> to do the comparisons (some other popular implementations do that). This diversity of approaches makes it difficult to write portable programs that attempt to instantiate the num_get template on user-defined types.
[Kona: the heart of the problem is that we're theoretically supposed to use traits classes for all fundamental character operations like assignment and comparison, but facets don't have traits parameters. This is a fundamental design flaw and it appears all over the place, not just in this one place. It's not clear what the correct solution is, but a thorough review of facets and traits is in order. The LWG considered and rejected the possibility of changing numeric facets to use narrowing instead of widening. This may be a good idea for other reasons (see issue 459), but it doesn't solve the problem raised by this issue. Whether we use widen or narrow the num_get facet still has no idea which traits class the user wants to use for the comparison, because only streams, not facets, are passed traits classes. The standard does not require that two different traits classes with the same char_type must necessarily have the same behavior.]
Informally, one possibility: require that some of the basic character operations, such as eq, lt, and assign, must behave the same way for all traits classes with the same char_type. If we accept that limitation on traits classes, then the facet could reasonably be required to use char_traits<charT>.
Proposed resolution:
Section: 26.5.2.4 [valarray.sub] Status: Open Submitter: Martin Sebor Date: 2003-09-18
View all issues with Open status.
Discussion:
The standard fails to specify the behavior of valarray::operator[](slice) and other valarray subset operations when they are passed an "invalid" slice object, i.e., either a slice that doesn't make sense at all (e.g., slice (0, 1, 0) or one that doesn't specify a valid subset of the valarray object (e.g., slice (2, 1, 1) for a valarray of size 1).
[Kona: the LWG believes that invalid slices should invoke undefined behavior. Valarrays are supposed to be designed for high performance, so we don't want to require specific checking. We need wording to express this decision.]
[ Bellevue: ]
Please note that the standard also fails to specify the behavior of slice_array and gslice_array in the valid case. Bill Plauger will endeavor to provide revised wording for slice_array and gslice_array.
[ post-Bellevue: Bill provided wording. ]
Proposed resolution:
Insert after 26.5.2.4 [valarray.sub], paragraph 1:
The member operator is overloaded to provide several ways to select sequences of elements from among those controlled by *this. The first group of five member operators work in conjunction with various overloads of operator= (and other assigning operators) to allow selective replacement (slicing) of the controlled sequence. The selected elements must exist.
The first member operator selects element off. For example:
valarray<char> v0("abcdefghijklmnop", 16); v0[3] = 'A'; // v0 == valarray<char>("abcAefghijklmnop", 16)The second member operator selects those elements of the controlled sequence designated by slicearr. For example:
valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDE", 5); v0[slice(2, 5, 3)] = v1; // v0 == valarray<char>("abAdeBghCjkDmnEp", 16)The third member operator selects those elements of the controlled sequence designated by gslicearr. For example:
valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDEF", 6); const size_t lv[] = {2, 3}; const size_t dv[] = {7, 2}; const valarray<size_t> len(lv, 2), str(dv, 2); v0[gslice(3, len, str)] = v1; // v0 == valarray<char>("abcAeBgCijDlEnFp", 16)The fourth member operator selects those elements of the controlled sequence designated by boolarr. For example:
valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABC", 3); const bool vb[] = {false, false, true, true, false, true}; v0[valarray<bool>(vb, 6)] = v1; // v0 == valarray<char>("abABeCghijklmnop", 16)The fifth member operator selects those elements of the controlled sequence designated by indarr. For example:
valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDE", 5); const size_t vi[] = {7, 5, 2, 3, 8}; v0[valarray<size_t>(vi, 5)] = v1; // v0 == valarray<char>("abCDeBgAEjklmnop", 16)The second group of five member operators each construct an object that represents the value(s) selected. The selected elements must exist.
The sixth member operator returns the value of element off. For example:
valarray<char> v0("abcdefghijklmnop", 16); // v0[3] returns 'd'The seventh member operator returns an object of class valarray<Ty> containing those elements of the controlled sequence designated by slicearr. For example:
valarray<char> v0("abcdefghijklmnop", 16); // v0[slice(2, 5, 3)] returns valarray<char>("cfilo", 5)The eighth member operator selects those elements of the controlled sequence designated by gslicearr. For example:
valarray<char> v0("abcdefghijklmnop", 16); const size_t lv[] = {2, 3}; const size_t dv[] = {7, 2}; const valarray<size_t> len(lv, 2), str(dv, 2); // v0[gslice(3, len, str)] returns // valarray<char>("dfhkmo", 6)The ninth member operator selects those elements of the controlled sequence designated by boolarr. For example:
valarray<char> v0("abcdefghijklmnop", 16); const bool vb[] = {false, false, true, true, false, true}; // v0[valarray<bool>(vb, 6)] returns // valarray<char>("cdf", 3)The last member operator selects those elements of the controlled sequence designated by indarr. For example:
valarray<char> v0("abcdefghijklmnop", 16); const size_t vi[] = {7, 5, 2, 3, 8}; // v0[valarray<size_t>(vi, 5)] returns // valarray<char>("hfcdi", 5)
Section: 20.1.2 [allocator.requirements], 25 [algorithms] Status: Open Submitter: Matt Austern Date: 2003-09-20
View other active issues in [allocator.requirements].
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Discussion:
Clause 20.1.2 [allocator.requirements] paragraph 4 says that implementations are permitted to supply containers that are unable to cope with allocator instances and that container implementations may assume that all instances of an allocator type compare equal. We gave implementers this latitude as a temporary hack, and eventually we want to get rid of it. What happens when we're dealing with allocators that don't compare equal?
In particular: suppose that v1 and v2 are both objects of type vector<int, my_alloc> and that v1.get_allocator() != v2.get_allocator(). What happens if we write v1.swap(v2)? Informally, three possibilities:
1. This operation is illegal. Perhaps we could say that an implementation is required to check and to throw an exception, or perhaps we could say it's undefined behavior.
2. The operation performs a slow swap (i.e. using three invocations of operator=, leaving each allocator with its original container. This would be an O(N) operation.
3. The operation swaps both the vectors' contents and their allocators. This would be an O(1) operation. That is:
my_alloc a1(...);
my_alloc a2(...);
assert(a1 != a2);
vector<int, my_alloc> v1(a1);
vector<int, my_alloc> v2(a2);
assert(a1 == v1.get_allocator());
assert(a2 == v2.get_allocator());
v1.swap(v2);
assert(a1 == v2.get_allocator());
assert(a2 == v1.get_allocator());
[Kona: This is part of a general problem. We need a paper saying how to deal with unequal allocators in general.]
[pre-Sydney: Howard argues for option 3 in N1599. ]
[ 2007-01-12, Howard: This issue will now tend to come up more often with move constructors and move assignment operators. For containers, these members transfer resources (i.e. the allocated memory) just like swap. ]
[ Batavia: There is agreement to overload the container swap on the allocator's Swappable requirement using concepts. If the allocator supports Swappable, then container's swap will swap allocators, else it will perform a "slow swap" using copy construction and copy assignment. ]
Proposed resolution:
Section: 24.1 [iterator.concepts], 23.1 [container.requirements] Status: Open Submitter: Andy Koenig Date: 2003-12-16
View other active issues in [iterator.concepts].
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Discussion:
What requirements does the standard place on equality comparisons between iterators that refer to elements of different containers. For example, if v1 and v2 are empty vectors, is v1.end() == v2.end() allowed to yield true? Is it allowed to throw an exception?
The standard appears to be silent on both questions.
[Sydney: The intention is that comparing two iterators from different containers is undefined, but it's not clear if we say that, or even whether it's something we should be saying in clause 23 or in clause 24. Intuitively we might want to say that equality is defined only if one iterator is reachable from another, but figuring out how to say it in any sensible way is a bit tricky: reachability is defined in terms of equality, so we can't also define equality in terms of reachability. ]
Proposed resolution:
Section: 24.1.6 [random.access.iterators] Status: Open Submitter: Daniel Frey Date: 2004-02-27
View all other issues in [random.access.iterators].
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Discussion:
In 24.1.5 [lib.random.access.iterators], table 76 the operational semantics for the expression "r -= n" are defined as "return r += -n". This means, that the expression -n must be valid, which is not the case for unsigned types.
[ Sydney: Possibly not a real problem, since difference type is required to be a signed integer type. However, the wording in the standard may be less clear than we would like. ]
Proposed resolution:
To remove this limitation, I suggest to change the operational semantics for this column to:
{ Distance m = n;
if (m >= 0)
while (m--) --r;
else
while (m++) ++r;
return r; }
Section: 22.2.2.1.2 [facet.num.get.virtuals] Status: Open Submitter: Martin Sebor Date: 2004-03-16
View other active issues in [facet.num.get.virtuals].
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Discussion:
When parsing strings of wide-character digits, the standard requires the library to widen narrow-character "atoms" and compare the widened atoms against the characters that are being parsed. Simply narrowing the wide characters would be far simpler, and probably more efficient. The two choices are equivalent except in convoluted test cases, and many implementations already ignore the standard and use narrow instead of widen.
First, I disagree that using narrow() instead of widen() would necessarily have unfortunate performance implications. A possible implementation of narrow() that allows num_get to be implemented in a much simpler and arguably comparably efficient way as calling widen() allows, i.e. without making a virtual call to do_narrow every time, is as follows:
inline char ctype<wchar_t>::narrow (wchar_t wc, char dflt) const
{
const unsigned wi = unsigned (wc);
if (wi > UCHAR_MAX)
return typeid (*this) == typeid (ctype<wchar_t>) ?
dflt : do_narrow (wc, dflt);
if (narrow_ [wi] < 0) {
const char nc = do_narrow (wc, dflt);
if (nc == dflt)
return dflt;
narrow_ [wi] = nc;
}
return char (narrow_ [wi]);
}
Second, I don't think the change proposed in the issue (i.e., to use narrow() instead of widen() during Stage 2) would be at all drastic. Existing implementations with the exception of libstdc++ currently already use narrow() so the impact of the change on programs would presumably be isolated to just a single implementation. Further, since narrow() is not required to translate alternate wide digit representations such as those mentioned in issue 303 to their narrow equivalents (i.e., the portable source characters '0' through '9'), the change does not necessarily imply that these alternate digits would be treated as ordinary digits and accepted as part of numbers during parsing. In fact, the requirement in 22.2.1.1.2 [locale.ctype.virtuals], p13 forbids narrow() to translate an alternate digit character, wc, to an ordinary digit in the basic source character set unless the expression (ctype<charT>::is(ctype_base::digit, wc) == true) holds. This in turn is prohibited by the C standard (7.25.2.1.5, 7.25.2.1.5, and 5.2.1, respectively) for charT of either char or wchar_t.
[Sydney: To a large extent this is a nonproblem. As long as you're only trafficking in char and wchar_t we're only dealing with a stable character set, so you don't really need either 'widen' or 'narrow': can just use literals. Finally, it's not even clear whether widen-vs-narrow is the right question; arguably we should be using codecvt instead.]
Proposed resolution:
Change stage 2 so that implementations are permitted to use either technique to perform the comparison:
Section: D.9.1 [auto.ptr] Status: Open Submitter: Rani Sharoni Date: 2003-12-07
View all other issues in [auto.ptr].
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Discussion:
TC1 CWG DR #84 effectively made the template<class Y> operator auto_ptr<Y>() member of auto_ptr (20.4.5.3/4) obsolete.
The sole purpose of this obsolete conversion member is to enable copy initialization base from r-value derived (or any convertible types like cv-types) case:
#include <memory>
using std::auto_ptr;
struct B {};
struct D : B {};
auto_ptr<D> source();
int sink(auto_ptr<B>);
int x1 = sink( source() ); // #1 EDG - no suitable copy constructor
The excellent analysis of conversion operations that was given in the final auto_ptr proposal (http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/1997/N1128.pdf) explicitly specifies this case analysis (case 4). DR #84 makes the analysis wrong and actually comes to forbid the loophole that was exploited by the auto_ptr designers.
I didn't encounter any compliant compiler (e.g. EDG, GCC, BCC and VC) that ever allowed this case. This is probably because it requires 3 user defined conversions and in fact current compilers conform to DR #84.
I was surprised to discover that the obsolete conversion member actually has negative impact of the copy initialization base from l-value derived case:
auto_ptr<D> dp; int x2 = sink(dp); // #2 EDG - more than one user-defined conversion applies
I'm sure that the original intention was allowing this initialization using the template<class Y> auto_ptr(auto_ptr<Y>& a) constructor (20.4.5.1/4) but since in this copy initialization it's merely user defined conversion (UDC) and the obsolete conversion member is UDC with the same rank (for the early overloading stage) there is an ambiguity between them.
Removing the obsolete member will have impact on code that explicitly invokes it:
int y = sink(source().operator auto_ptr<B>());
IMHO no one ever wrote such awkward code and the reasonable workaround for #1 is:
int y = sink( auto_ptr<B>(source()) );
I was even more surprised to find out that after removing the obsolete conversion member the initialization was still ill-formed: int x3 = sink(dp); // #3 EDG - no suitable copy constructor
This copy initialization semantically requires copy constructor which means that both template conversion constructor and the auto_ptr_ref conversion member (20.4.5.3/3) are required which is what was explicitly forbidden in DR #84. This is a bit amusing case in which removing ambiguity results with no candidates.
I also found exception safety issue with auto_ptr related to auto_ptr_ref:
int f(auto_ptr<B>, std::string); auto_ptr<B> source2(); // string constructor throws while auto_ptr_ref // "holds" the pointer int x4 = f(source2(), "xyz"); // #4
The theoretic execution sequence that will cause a leak:
According to 20.4.5.3/3 and 20.4.5/2 the auto_ptr_ref conversion member returns auto_ptr_ref<Y> that holds *this and this is another defect since the type of *this is auto_ptr<X> where X might be different from Y. Several library vendors (e.g. SGI) implement auto_ptr_ref<Y> with Y* as member which is much more reasonable. Other vendor implemented auto_ptr_ref as defectively required and it results with awkward and catastrophic code: int oops = sink(auto_ptr<B>(source())); // warning recursive on all control paths
Dave Abrahams noticed that there is no specification saying that auto_ptr_ref copy constructor can't throw.
My proposal comes to solve all the above issues and significantly simplify auto_ptr implementation. One of the fundamental requirements from auto_ptr is that it can be constructed in an intuitive manner (i.e. like ordinary pointers) but with strict ownership semantics which yield that source auto_ptr in initialization must be non-const. My idea is to add additional constructor template with sole propose to generate ill-formed, diagnostic required, instance for const auto_ptr arguments during instantiation of declaration. This special constructor will not be instantiated for other types which is achievable using 14.8.2/2 (SFINAE). Having this constructor in hand makes the constructor template<class Y> auto_ptr(auto_ptr<Y> const&) legitimate since the actual argument can't be const yet non const r-value are acceptable.
This implementation technique makes the "private auxiliary class" auto_ptr_ref obsolete and I found out that modern C++ compilers (e.g. EDG, GCC and VC) consume the new implementation as expected and allow all intuitive initialization and assignment cases while rejecting illegal cases that involve const auto_ptr arguments.
The proposed auto_ptr interface:
namespace std {
template<class X> class auto_ptr {
public:
typedef X element_type;
// 20.4.5.1 construct/copy/destroy:
explicit auto_ptr(X* p=0) throw();
auto_ptr(auto_ptr&) throw();
template<class Y> auto_ptr(auto_ptr<Y> const&) throw();
auto_ptr& operator=(auto_ptr&) throw();
template<class Y> auto_ptr& operator=(auto_ptr<Y>) throw();
~auto_ptr() throw();
// 20.4.5.2 members:
X& operator*() const throw();
X* operator->() const throw();
X* get() const throw();
X* release() throw();
void reset(X* p=0) throw();
private:
template<class U>
auto_ptr(U& rhs, typename
unspecified_error_on_const_auto_ptr<U>::type = 0);
};
}
One compliant technique to implement the unspecified_error_on_const_auto_ptr helper class is using additional private auto_ptr member class template like the following:
template<typename T> struct unspecified_error_on_const_auto_ptr;
template<typename T>
struct unspecified_error_on_const_auto_ptr<auto_ptr<T> const>
{ typedef typename auto_ptr<T>::const_auto_ptr_is_not_allowed type; };
There are other techniques to implement this helper class that might work better for different compliers (i.e. better diagnostics) and therefore I suggest defining its semantic behavior without mandating any specific implementation. IMO, and I didn't found any compiler that thinks otherwise, 14.7.1/5 doesn't theoretically defeat the suggested technique but I suggest verifying this with core language experts.
Further changes in standard text:
Remove section 20.4.5.3
Change 20.4.5/2 to read something like: Initializing auto_ptr<X> from const auto_ptr<Y> will result with unspecified ill-formed declaration that will require unspecified diagnostic.
Change 20.4.5.1/4,5,6 to read:
template<class Y> auto_ptr(auto_ptr<Y> const& a) throw();
4 Requires: Y* can be implicitly converted to X*.
5 Effects: Calls const_cast<auto_ptr<Y>&>(a).release().
6 Postconditions: *this holds the pointer returned from a.release().
Change 20.4.5.1/10
template<class Y> auto_ptr& operator=(auto_ptr<Y> a) throw();
10 Requires: Y* can be implicitly converted to X*. The expression delete get() is well formed.
LWG TC DR #127 is obsolete.
Notice that the copy constructor and copy assignment operator should remain as before and accept non-const auto_ptr& since they have effect on the form of the implicitly declared copy constructor and copy assignment operator of class that contains auto_ptr as member per 12.8/5,10:
struct X {
// implicit X(X&)
// implicit X& operator=(X&)
auto_ptr<D> aptr_;
};
In most cases this indicates about sloppy programming but preserves the current auto_ptr behavior.
Dave Abrahams encouraged me to suggest fallback implementation in case that my suggestion that involves removing of auto_ptr_ref will not be accepted. In this case removing the obsolete conversion member to auto_ptr<Y> and 20.4.5.3/4,5 is still required in order to eliminate ambiguity in legal cases. The two constructors that I suggested will co exist with the current members but will make auto_ptr_ref obsolete in initialization contexts. auto_ptr_ref will be effective in assignment contexts as suggested in DR #127 and I can't see any serious exception safety issues in those cases (although it's possible to synthesize such). auto_ptr_ref<X> semantics will have to be revised to say that it strictly holds pointer of type X and not reference to an auto_ptr for the favor of cases in which auto_ptr_ref<Y> is constructed from auto_ptr<X> in which X is different from Y (i.e. assignment from r-value derived to base).
[Redmond: punt for the moment. We haven't decided yet whether we want to fix auto_ptr for C++-0x, or remove it and replace it with move_ptr and unique_ptr.]
[ Oxford 2007: Recommend NAD. We're just going to deprecate it. It still works for simple use cases and people know how to deal with it. Going forward unique_ptr is the recommended tool. ]
[ 2007-11-09: Reopened at the request of David Abrahams, Alisdair Meredith and Gabriel Dos Reis. ]
Proposed resolution:
Change the synopsis in D.9.1 [auto.ptr]:
namespace std {
template <class Y> struct auto_ptr_ref {};
// exposition only
template <class T> struct constant_object;
// exposition only
template <class T>
struct cannot_transfer_ownership_from
: constant_object<T> {};
template <class X> class auto_ptr {
public:
typedef X element_type;
// D.9.1.1 construct/copy/destroy:
explicit auto_ptr(X* p =0) throw();
auto_ptr(auto_ptr&) throw();
template<class Y> auto_ptr(auto_ptr<Y> const&) throw();
auto_ptr& operator=(auto_ptr&) throw();
template<class Y> auto_ptr& operator=(auto_ptr<Y>&) throw();
auto_ptr& operator=(auto_ptr_ref<X> r) throw();
~auto_ptr() throw();
// D.9.1.2 members:
X& operator*() const throw();
X* operator->() const throw();
X* get() const throw();
X* release() throw();
void reset(X* p =0) throw();
// D.9.1.3 conversions:
auto_ptr(auto_ptr_ref<X>) throw();
template<class Y> operator auto_ptr_ref<Y>() throw();
template<class Y> operator auto_ptr<Y>() throw();
// exposition only
template<class U>
auto_ptr(U& rhs, typename cannot_transfer_ownership_from<U>::error = 0);
};
template <> class auto_ptr<void>
{
public:
typedef void element_type;
};
}
Remove D.9.1.3 [auto.ptr.conv].
Change D.9.1 [auto.ptr], p3:
The auto_ptr provides a semantics of strict ownership. An auto_ptr owns the object it holds a pointer to. Copying an auto_ptr copies the pointer and transfers ownership to the destination. If more than one auto_ptr owns the same object at the same time the behavior of the program is undefined. Templates constant_object and cannot_transfer_ownership_from, and the final constructor of auto_ptr are for exposition only. For any types X and Y, initializing auto_ptr<X> from const auto_ptr<Y> is ill-formed, diagnostic required. [Note: The uses of auto_ptr include providing temporary exception-safety for dynamically allocated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function. auto_ptr does not meet the CopyConstructible and Assignable requirements for Standard Library container elements and thus instantiating a Standard Library container with an auto_ptr results in undefined behavior. -- end note]
Change D.9.1.1 [auto.ptr.cons], p5:
template<class Y> auto_ptr(auto_ptr<Y> const& a) throw();Requires: Y* can be implicitly converted to X*.
Effects: Calls const_cast<auto_ptr<Y>&>(a).release().
Postconditions: *this holds the pointer returned from a.release().
Change D.9.1.1 [auto.ptr.cons], p10:
template<class Y> auto_ptr& operator=(auto_ptr<Y>&a) throw();Requires: Y* can be implicitly converted to X*. The expression delete get() is well formed.
Effects: Calls reset(a.release()).
Returns: *this.
Section: 18.6.1 [type.info] Status: Open Submitter: Martin Sebor Date: 2004-06-28
View all other issues in [type.info].
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Discussion:
[lib.exception] specifies the following:
exception (const exception&) throw();
exception& operator= (const exception&) throw();
-4- Effects: Copies an exception object.
-5- Notes: The effects of calling what() after assignment
are implementation-defined.
First, does the Note only apply to the assignment operator? If so, what are the effects of calling what() on a copy of an object? Is the returned pointer supposed to point to an identical copy of the NTBS returned by what() called on the original object or not?
Second, is this Note intended to extend to all the derived classes in section 19? I.e., does the standard provide any guarantee for the effects of what() called on a copy of any of the derived class described in section 19?
Finally, if the answer to the first question is no, I believe it constitutes a defect since throwing an exception object typically implies invoking the copy ctor on the object. If the answer is yes, then I believe the standard ought to be clarified to spell out exactly what the effects are on the copy (i.e., after the copy ctor was called).
[Redmond: Yes, this is fuzzy. The issue of derived classes is fuzzy too.]
[ Batavia: Howard provided wording. ]
[ Bellevue: ]
Eric concerned this is unimplementable, due to nothrow guarantees. Suggested implementation would involve reference counting.
Is the implied reference counting subtle enough to call out a note on implementation? Probably not.
If reference counting required, could we tighten specification further to require same pointer value? Probably an overspecification, especially if exception classes defer evalutation of final string to calls to what().
Remember issue moved open and not resolved at Batavia, but cannot remember who objected to canvas a disenting opinion - please speak up if you disagree while reading these minutes!
Move to Ready as we are accepting words unmodified.
[ Sophia Antipolis: ]
The issue was pulled from Ready. It needs to make clear that only homogenous copying is intended to be supported. Not coping from a dervied to a base.
Proposed resolution:
Change 18.7.1 [exception] to:
exception(const exception& e) throw(); exception& operator=(const exception& e) throw();-4- Effects: Copies an exception object.
-5- Remarks: The effects of calling what() after assignment are implementation-defined.-5- Throws: Nothing. This also applies to all standard library-defined classes that derive from exception.
-7- Postcondition: strcmp(what(), e.what()) == 0. This also applies to all standard library-defined classes that derive from exception.
Section: 22.2.1.1 [locale.ctype] Status: Open Submitter: Martin Sebor Date: 2004-07-01
View all issues with Open status.
Discussion:
Most ctype member functions come in two forms: one that operates on a single character at a time and another form that operates on a range of characters. Both forms are typically described by a single Effects and/or Returns clause.
The Returns clause of each of the single-character non-virtual forms suggests that the function calls the corresponding single character virtual function, and that the array form calls the corresponding virtual array form. Neither of the two forms of each virtual member function is required to be implemented in terms of the other.
There are three problems:
1. One is that while the standard does suggest that each non-virtual member function calls the corresponding form of the virtual function, it doesn't actually explicitly require it.
Implementations that cache results from some of the virtual member functions for some or all values of their arguments might want to call the array form from the non-array form the first time to fill the cache and avoid any or most subsequent virtual calls. Programs that rely on each form of the virtual function being called from the corresponding non-virtual function will see unexpected behavior when using such implementations.
2. The second problem is that either form of each of the virtual functions can be overridden by a user-defined function in a derived class to return a value that is different from the one produced by the virtual function of the alternate form that has not been overriden.
Thus, it might be possible for, say, ctype::widen(c) to return one value, while for ctype::widen(&c, &c + 1, &wc) to set wc to another value. This is almost certainly not intended. Both forms of every function should be required to return the same result for the same character, otherwise the same program using an implementation that calls one form of the functions will behave differently than when using another implementation that calls the other form of the function "under the hood."
3. The last problem is that the standard text fails to specify whether one form of any of the virtual functions is permitted to be implemented in terms of the other form or not, and if so, whether it is required or permitted to call the overridden virtual function or not.
Thus, a program that overrides one of the virtual functions so that it calls the other form which then calls the base member might end up in an infinite loop if the called form of the base implementation of the function in turn calls the other form.
Lillehammer: Part of this isn't a real problem. We already talk about caching. 22.1.1/6 But part is a real problem. ctype virtuals may call each other, so users don't know which ones to override to avoid avoid infinite loops.
This is a problem for all facet virtuals, not just ctype virtuals, so we probably want a blanket statement in clause 22 for all facets. The LWG is leaning toward a blanket prohibition, that a facet's virtuals may never call each other. We might want to do that in clause 27 too, for that matter. A review is necessary. Bill will provide wording.
Proposed resolution:
Section: 24.1.3 [output.iterators] Status: Open Submitter: Chris Jefferson Date: 2004-10-13
View all other issues in [output.iterators].
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Discussion:
The note on 24.1.2 Output iterators insufficently limits what can be performed on output iterators. While it requires that each iterator is progressed through only once and that each iterator is written to only once, it does not require the following things:
Note: Here it is assumed that x is an output iterator of type X which has not yet been assigned to.
a) That each value of the output iterator is written to: The standard allows: ++x; ++x; ++x;
b) That assignments to the output iterator are made in order X a(x); ++a; *a=1; *x=2; is allowed
c) Chains of output iterators cannot be constructed: X a(x); ++a; X b(a); ++b; X c(b); ++c; is allowed, and under the current wording (I believe) x,a,b,c could be written to in any order.
I do not believe this was the intension of the standard?
[Lillehammer: Real issue. There are lots of constraints we intended but didn't specify. Should be solved as part of iterator redesign.]
Proposed resolution:
Section: 23 [containers], 24 [iterators], 25 [algorithms] Status: Open Submitter: Thomas Mang Date: 2004-12-12
View other active issues in [containers].
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Discussion:
Various clauses other than clause 25 make use of iterator arithmetic not supported by the iterator category in question. Algorithms in clause 25 are exceptional because of 25 [lib.algorithms], paragraph 9, but this paragraph does not provide semantics to the expression "iterator - n", where n denotes a value of a distance type between iterators.
1) Examples of current wording:
Current wording outside clause 25:
23.2.2.4 [lib.list.ops], paragraphs 19-21: "first + 1", "(i - 1)", "(last - first)" 23.3.1.1 [lib.map.cons], paragraph 4: "last - first" 23.3.2.1 [lib.multimap.cons], paragraph 4: "last - first" 23.3.3.1 [lib.set.cons], paragraph 4: "last - first" 23.3.4.1 [lib.multiset.cons], paragraph 4: "last - first" 24.4.1 [lib.reverse.iterators], paragraph 1: "(i - 1)"
[Important note: The list is not complete, just an illustration. The same issue might well apply to other paragraphs not listed here.]
None of these expressions is valid for the corresponding iterator category.
Current wording in clause 25:
25.1.1 [lib.alg.foreach], paragraph 1: "last - 1" 25.1.3 [lib.alg.find.end], paragraph 2: "[first1, last1 - (last2-first2))" 25.2.8 [lib.alg.unique], paragraph 1: "(i - 1)" 25.2.8 [lib.alg.unique], paragraph 5: "(i - 1)"
However, current wording of 25 [lib.algorithms], paragraph 9 covers neither of these four cases:
Current wording of 25 [lib.algorithms], paragraph 9:
"In the description of the algorithms operator + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of a+n is the same as that of
{X tmp = a;
advance(tmp, n);
return tmp;
}
and that of b-a is the same as of return distance(a, b)"
This paragrpah does not take the expression "iterator - n" into account, where n denotes a value of a distance type between two iterators [Note: According to current wording, the expression "iterator - n" would be resolved as equivalent to "return distance(n, iterator)"]. Even if the expression "iterator - n" were to be reinterpreted as equivalent to "iterator + -n" [Note: This would imply that "a" and "b" were interpreted implicitly as values of iterator types, and "n" as value of a distance type], then 24.3.4/2 interfers because it says: "Requires: n may be negative only for random access and bidirectional iterators.", and none of the paragraphs quoted above requires the iterators on which the algorithms operate to be of random access or bidirectional category.
2) Description of intended behavior:
For the rest of this Defect Report, it is assumed that the expression "iterator1 + n" and "iterator1 - iterator2" has the semantics as described in current 25 [lib.algorithms], paragraph 9, but applying to all clauses. The expression "iterator1 - n" is equivalent to an result-iterator for which the expression "result-iterator + n" yields an iterator denoting the same position as iterator1 does. The terms "iterator1", "iterator2" and "result-iterator" shall denote the value of an iterator type, and the term "n" shall denote a value of a distance type between two iterators.
All implementations known to the author of this Defect Report comply with these assumptions. No impact on current code is expected.
3) Proposed fixes:
Change 25 [lib.algorithms], paragraph 9 to:
"In the description of the algorithms operator + and - are used for some of the iterator categories for which they do not have to be defined. In this paragraph, a and b denote values of an iterator type, and n denotes a value of a distance type between two iterators. In these cases the semantics of a+n is the same as that of
{X tmp = a;
advance(tmp, n);
return tmp;
}
,the semantics of a-n denotes the value of an iterator i for which the following condition holds: advance(i, n) == a, and that of b-a is the same as of return distance(a, b)".
Comments to the new wording:
a) The wording " In this paragraph, a and b denote values of an iterator type, and n denotes a value of a distance type between two iterators." was added so the expressions "b-a" and "a-n" are distinguished regarding the types of the values on which they operate. b) The wording ",the semantics of a-n denotes the value of an iterator i for which the following condition holds: advance(i, n) == a" was added to cover the expression 'iterator - n'. The wording "advance(i, n) == a" was used to avoid a dependency on the semantics of a+n, as the wording "i + n == a" would have implied. However, such a dependency might well be deserved. c) DR 225 is not considered in the new wording.
Proposed fixes regarding invalid iterator arithmetic expressions outside clause 25:
Either a) Move modified 25 [lib.algorithms], paragraph 9 (as proposed above) before any current invalid iterator arithmetic expression. In that case, the first sentence of 25 [lib.algorithms], paragraph 9, need also to be modified and could read: "For the rest of this International Standard, ...." / "In the description of the following clauses including this ...." / "In the description of the text below ..." etc. - anyways substituting the wording "algorithms", which is a straight reference to clause 25. In that case, 25 [lib.algorithms] paragraph 9 will certainly become obsolete. Alternatively, b) Add an appropiate paragraph similar to resolved 25 [lib.algorithms], paragraph 9, to the beginning of each clause containing invalid iterator arithmetic expressions. Alternatively, c) Fix each paragraph (both current wording and possible resolutions of DRs) containing invalid iterator arithmetic expressions separately.
5) References to other DRs:
See DR 225. See DR 237. The resolution could then also read "Linear in last - first".
[ Bellevue: ]
Keep open and ask Bill to provide wording.
Proposed resolution:
[Lillehammer: Minor issue, but real. We have a blanket statement about this in 25/11. But (a) it should be in 17, not 25; and (b) it's not quite broad enough, because there are some arithmetic expressions it doesn't cover. Bill will provide wording.]
Section: 25.2.13 [alg.partitions] Status: Open Submitter: Sean Parent, Joe Gottman Date: 2005-05-04
View all issues with Open status.
Discussion:
Problem: The iterator requirements for partition() and stable_partition() [25.2.12] are listed as BidirectionalIterator, however, there are efficient algorithms for these functions that only require ForwardIterator that have been known since before the standard existed. The SGI implementation includes these (see http://www.sgi.com/tech/stl/partition.html and http://www.sgi.com/tech/stl/stable_partition.html).
Proposed resolution:
Change 25.2.12 from
template<class BidirectionalIterator, class Predicate>
BidirectionalIterator partition(BidirectionalIterato r first,
BidirectionalIterator last,
Predicate pred);
to
template<class ForwardIterator, class Predicate>
ForwardIterator partition(ForwardIterator first,
ForwardIterator last,
Predicate pred);
Change the complexity from
At most (last - first)/2 swaps are done. Exactly (last - first) applications of the predicate are done.
to
If ForwardIterator is a bidirectional_iterator, at most (last - first)/2 swaps are done; otherwise at most (last - first) swaps are done. Exactly (last - first) applications of the predicate are done.
Rationale:
Partition is a "foundation" algorithm useful in many contexts (like sorting as just one example) - my motivation for extending it to include forward iterators is slist - without this extension you can't partition an slist (without writing your own partition). Holes like this in the standard library weaken the argument for generic programming (ideally I'd be able to provide a library that would refine std::partition() to other concepts without fear of conflicting with other libraries doing the same - but that is a digression). I consider the fact that partition isn't defined to work for ForwardIterator a minor embarrassment.
[Mont Tremblant: Moved to Open, request motivation and use cases by next meeting. Sean provided further rationale by post-meeting mailing.]
Section: 22.1.1.1.1 [locale.category] Status: Open Submitter: Christopher Conrade Zseleghovski Date: 2005-06-07
View all other issues in [locale.category].
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Discussion:
Motivation:
This requirement seems obvious to me, it is the essence of code modularity. I have complained to Mr. Plauger that the Dinkumware library does not observe this principle but he objected that this behaviour is not covered in the standard.
Proposed resolution:
Append the following point to 22.1.1.1.1:
6. The implementation of a facet of Table 52 parametrized with an InputIterator/OutputIterator should use that iterator only as character source/sink respectively. For a *_get facet, it means that the value received depends only on the sequence of input characters and not on how they are accessed. For a *_put facet, it means that the sequence of characters output depends only on the value to be formatted and not of how the characters are stored.
[ Berlin: Moved to Open, Need to clean up this area to make it clear locales don't have to contain open ended sets of facets. Jack, Howard, Bill. ]
Section: 22.2 [locale.categories] Status: Open Submitter: P.J. Plauger Date: 2005-06-20
View other active issues in [locale.categories].
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Discussion:
a) In 22.2.1.1 para. 2 we refer to "the instantiations required in Table 51" to refer to the facet *objects* associated with a locale. And we almost certainly mean just those associated with the default or "C" locale. Otherwise, you can't switch to a locale that enforces a different mapping between narrow and wide characters, or that defines additional uppercase characters.
b) 22.2.1.5 para. 3 (codecvt) has the same issues.
c) 22.2.1.5.2 (do_unshift) is even worse. It *forbids* the generation of a homing sequence for the basic character set, which might very well need one.
d) 22.2.1.5.2 (do_length) likewise dictates that the default mapping between wide and narrow characters be taken as one-for-one.
e) 22.2.2 para. 2 (num_get/put) is both muddled and vacuous, as far as I can tell. The muddle is, as before, calling Table 51 a list of instantiations. But the constraint it applies seems to me to cover *all* defined uses of num_get/put, so why bother to say so?
f) 22.2.3.1.2 para. 1(do_decimal_point) says "The required instantiations return '.' or L'.'.) Presumably this means "as appropriate for the character type. But given the vague definition of "required" earlier, this overrules *any* change of decimal point for non "C" locales. Surely we don't want to do that.
g) 22.2.3.1.2 para. 2 (do_thousands_sep) says "The required instantiations return ',' or L','.) As above, this probably means "as appropriate for the character type. But this overrules the "C" locale, which requires *no* character ('\0') for the thousands separator. Even if we agree that we don't mean to block changes in decimal point or thousands separator, we should also eliminate this clear incompatibility with C.
h) 22.2.3.1.2 para. 2 (do_grouping) says "The required instantiations return the empty string, indicating no grouping." Same considerations as for do_decimal_point.
i) 22.2.4.1 para. 1 (collate) refers to "instantiations required in Table 51". Same bad jargon.
j) 22.2.4.1.2 para. 1 (do_compare) refers to "instantiations required in Table 51". Same bad jargon.
k) 22.2.5 para. 1 (time_get/put) uses the same muddled and vacuous as num_get/put.
l) 22.2.6 para. 2 (money_get/put) uses the same muddled and vacuous as num_get/put.
m) 22.2.6.3.2 (do_pos/neg_format) says "The instantiations required in Table 51 ... return an object of type pattern initialized to {symbol, sign, none, value}." This once again *overrides* the "C" locale, as well as any other locale."
3) We constrain the use_facet calls that can be made by num_get/put, so why don't we do the same for money_get/put? Or for any of the other facets, for that matter?
4) As an almost aside, we spell out when a facet needs to use the ctype facet, but several also need to use a codecvt facet and we don't say so.
[ Berlin: Bill to provide wording. ]
Proposed resolution:
Section: 28 [re] Status: Open Submitter: Eric Niebler Date: 2005-07-01
View all other issues in [re].
View all issues with Open status.
Discussion:
A problem with TR1 regex is currently being discussed on the Boost developers list. It involves the handling of case-insensitive matching of character ranges such as [Z-a]. The proper behavior (according to the ECMAScript standard) is unimplementable given the current specification of the TR1 regex_traits<> class template. John Maddock, the author of the TR1 regex proposal, agrees there is a problem. The full discussion can be found at http://lists.boost.org/boost/2005/06/28850.php (first message copied below). We don't have any recommendations as yet.
-- Begin original message --
The situation of interest is described in the ECMAScript specification (ECMA-262), section 15.10.2.15:
"Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case ASCII letters as well as the symbols [, \, ], ^, _, and `."
A more interesting case is what should happen when doing a case-insentitive match on a range such as [Z-a]. It should match z, Z, a, A and the symbols [, \, ], ^, _, and `. This is not what happens with Boost.Regex (it throws an exception from the regex constructor).
The tough pill to swallow is that, given the specification in TR1, I don't think there is any effective way to handle this situation. According to the spec, case-insensitivity is handled with regex_traits<>::translate_nocase(CharT) -- two characters are equivalent if they compare equal after both are sent through the translate_nocase function. But I don't see any way of using this translation function to make character ranges case-insensitive. Consider the difficulty of detecting whether "z" is in the range [Z-a]. Applying the transformation to "z" has no effect (it is essentially std::tolower). And we're not allowed to apply the transformation to the ends of the range, because as ECMA-262 says, "the case of the two ends of a range is significant."
So AFAICT, TR1 regex is just broken, as is Boost.Regex. One possible fix is to redefine translate_nocase to return a string_type containing all the characters that should compare equal to the specified character. But this function is hard to implement for Unicode, and it doesn't play nice with the existing ctype facet. What a mess!
-- End original message --
[ John Maddock adds: ]
One small correction, I have since found that ICU's regex package does implement this correctly, using a similar mechanism to the current TR1.Regex.
Given an expression [c1-c2] that is compiled as case insensitive it:
Enumerates every character in the range c1 to c2 and converts it to it's case folded equivalent. That case folded character is then used a key to a table of equivalence classes, and each member of the class is added to the list of possible matches supported by the character-class. This second step isn't possible with our current traits class design, but isn't necessary if the input text is also converted to a case-folded equivalent on the fly.
ICU applies similar brute force mechanisms to character classes such as [[:lower:]] and [[:word:]], however these are at least cached, so the impact is less noticeable in this case.
Quick and dirty performance comparisons show that expressions such as "[X-\\x{fff0}]+" are indeed very slow to compile with ICU (about 200 times slower than a "normal" expression). For an application that uses a lot of regexes this could have a noticeable performance impact. ICU also has an advantage in that it knows the range of valid characters codes: code points outside that range are assumed not to require enumeration, as they can not be part of any equivalence class. I presume that if we want the TR1.Regex to work with arbitrarily large character sets enumeration really does become impractical.
Finally note that Unicode has:
Three cases (upper, lower and title). One to many, and many to one case transformations. Character that have context sensitive case translations - for example an uppercase sigma has two different lowercase forms - the form chosen depends on context(is it end of a word or not), a caseless match for an upper case sigma should match either of the lower case forms, which is why case folding is often approximated by tolower(toupper(c)).
Probably we need some way to enumerate character equivalence classes, including digraphs (either as a result or an input), and some way to tell whether the next character pair is a valid digraph in the current locale.
Hoping this doesn't make this even more complex that it was already,
[ Portland: Alisdair: Detect as invalid, throw an exception. Pete: Possible general problem with case insensitive ranges. ]
Proposed resolution:
Section: 26.6.3 [partial.sum] Status: Open Submitter: Marc Schoolderman Date: 2006-02-06
View all issues with Open status.
Discussion:
There are some problems in the definition of partial_sum and adjacent_difference in 26.4 [lib.numeric.ops]
Unlike accumulate and inner_product, these functions are not parametrized on a "type T", instead, 26.4.3 [lib.partial.sum] simply specifies the effects clause as;
Assigns to every element referred to by iterator i in the range [result,result + (last - first)) a value correspondingly equal to
((...(* first + *( first + 1)) + ...) + *( first + ( i - result )))
And similarly for BinaryOperation. Using just this definition, it seems logical to expect that:
char i_array[4] = { 100, 100, 100, 100 };
int o_array[4];
std::partial_sum(i_array, i_array+4, o_array);
Is equivalent to
int o_array[4] = { 100, 100+100, 100+100+100, 100+100+100+100 };
i.e. 100, 200, 300, 400, with addition happening in the result type, int.
Yet all implementations I have tested produce 100, -56, 44, -112, because they are using an accumulator of the InputIterator's value_type, which in this case is char, not int.
The issue becomes more noticeable when the result of the expression *i + *(i+1) or binary_op(*i, *i-1) can't be converted to the value_type. In a contrived example:
enum not_int { x = 1, y = 2 };
...
not_int e_array[4] = { x, x, y, y };
std::partial_sum(e_array, e_array+4, o_array);
Is it the intent that the operations happen in the input type, or in the result type?
If the intent is that operations happen in the result type, something like this should be added to the "Requires" clause of 26.4.3/4 [lib.partial.sum]:
The type of *i + *(i+1) or binary_op(*i, *(i+1)) shall meet the requirements of CopyConstructible (20.1.3) and Assignable (23.1) types.
(As also required for T in 26.4.1 [lib.accumulate] and 26.4.2 [lib.inner.product].)
The "auto initializer" feature proposed in N1894 is not required to implement partial_sum this way. The 'narrowing' behaviour can still be obtained by using the std::plus<> function object.
If the intent is that operations happen in the input type, then something like this should be added instead;
The type of *first shall meet the requirements of CopyConstructible (20.1.3) and Assignable (23.1) types. The result of *i + *(i+1) or binary_op(*i, *(i+1)) shall be convertible to this type.
The 'widening' behaviour can then be obtained by writing a custom proxy iterator, which is somewhat involved.
In both cases, the semantics should probably be clarified.
26.4.4 [lib.adjacent.difference] is similarly underspecified, although all implementations seem to perform operations in the 'result' type:
unsigned char i_array[4] = { 4, 3, 2, 1 };
int o_array[4];
std::adjacent_difference(i_array, i_array+4, o_array);
o_array is 4, -1, -1, -1 as expected, not 4, 255, 255, 255.
In any case, adjacent_difference doesn't mention the requirements on the value_type; it can be brought in line with the rest of 26.4 [lib.numeric.ops] by adding the following to 26.4.4/2 [lib.adjacent.difference]:
The type of *first shall meet the requirements of CopyConstructible (20.1.3) and Assignable (23.1) types."
[ Berlin: Giving output iterator's value_types very controversial. Suggestion of adding signatures to allow user to specify "accumulator". ]
[ Bellevue: ]
The intent of the algorithms is to perform their calculations using the type of the input iterator. Proposed wording provided.
[ Sophia Antipolis: ]
We did not agree that the proposed resolution was correct. For example, when the arguments are types (float*, float*, double*), the highest-quality solution would use double as the type of the accumulator. If the intent of the wording is to require that the type of the accumulator must be the input_iterator's value_type, the wording should specify it.
Proposed resolution:
Add to section 26.6.3 [partial.sum] paragraph 4 the following two sentences:
The type of *first shall meet the requirements of CopyConstructible? (20.1.3?) and Assignable (23.1?) types. The result of *i + *(i+1) or binary_op(*i, *(i+1)) shall be convertible to this type.
Add to section 26.6.4 [adjacent.difference] paragraph 2 the following sentence:
The type of *first shall meet the requirements of CopyConstructible? (20.1.3) and Assignable (23.1) types.
Section: TR1 5.1.1 [tr.rand.req] Status: Open Submitter: Matt Austern Date: 2006-01-10
View all issues with Open status.
Discussion:
The TR sneaks in two new integer types, _Longlong and _Ulonglong, in [tr.c99]. The rest of the TR should use that type. I believe this affects two places. First, the random number requirements, 5.1.1/10-11, lists all of the types with which template parameters named IntType and UIntType may be instantiated. _Longlong (or "long long", assuming it is added to C++0x) should be added to the IntType list, and UIntType (again, or "unsigned long long") should be added to the UIntType list. Second, 6.3.2 lists the types for which hash<> is required to be instantiable. _Longlong and _Ulonglong should be added to that list, so that people may use long long as a hash key.
Proposed resolution:
Section: 27.7.1.4 [stringbuf.virtuals] Status: Open Submitter: Martin Sebor Date: 2006-02-23
View all other issues in [stringbuf.virtuals].
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Discussion:
The effects of the seekpos() member function of
basic_stringbuf simply say that the function positions
the input and/or output sequences but fail to spell out exactly
how. This is in contrast to the detail in which seekoff()
is described.
Proposed resolution:
Change 27.7.1.3, p13 to read:
-13- Effects: Same as seekoff(off_type(sp), ios_base::beg, which).
Alters the stream position within the controlled sequences, if possible, to correspond to the stream position stored in sp (as described below).
If (which & ios_base::in) != 0, positions the input sequence.If (which & ios_base::out) != 0, positions the output sequence.If sp is an invalid stream position, or if the function positions neither sequence, the positioning operation fails. If sp has not been obtained by a previous successful call to one of the positioning functions (seekoff, seekpos, tellg, tellp) the effect is undefined.
[ Kona (2007): A pos_type is a position in a stream by definition, so there is no ambiguity as to what it means. Proposed Disposition: NAD ]
[ Post-Kona Martin adds: I'm afraid I disagree with the Kona '07 rationale for marking it NAD. The only text that describes precisely what it means to position the input or output sequence is in seekoff(). The seekpos() Effects clause is inadequate in comparison and the proposed resolution plugs the hole by specifying seekpos() in terms of seekoff(). ]
Section: 27.5.2.4.5 [streambuf.virt.put] Status: Open Submitter: Martin Sebor Date: 2006-02-23
View all issues with Open status.
Discussion:
streambuf::xsputn() is specified to have the effect of "writing up to n characters to the output sequence as if by repeated calls to sputc(c)."
Since sputc() is required to call overflow() when (pptr() == epptr()) is true, strictly speaking xsputn() should do the same. However, doing so would be suboptimal in some interesting cases, such as in unbuffered mode or when the buffer is basic_stringbuf.
Assuming calling overflow() is not really intended to be required and the wording is simply meant to describe the general effect of appending to the end of the sequence it would be worthwhile to mention in xsputn() that the function is not actually required to cause a call to overflow().
Proposed resolution:
Add the following sentence to the xsputn() Effects clause in 27.5.2.4.5, p1 (N1804):
-1- Effects: Writes up to n characters to the output sequence as if by repeated calls to sputc(c). The characters written are obtained from successive elements of the array whose first element is designated by s. Writing stops when either n characters have been written or a call to sputc(c) would return traits::eof(). It is uspecified whether the function calls overflow() when (pptr() == epptr()) becomes true or whether it achieves the same effects by other means.
In addition, I suggest to add a footnote to this function with the same text as Footnote 292 to make it extra clear that derived classes are permitted to override xsputn() for efficiency.
[ Kona (2007): We want to permit a streambuf that streams output directly to a device without making calls to sputc or overflow. We believe that has always been the intention of the committee. We believe that the proposed wording doesn't accomplish that. Proposed Disposition: Open ]
Section: TR1 8.16.4 [tr.c99.cmath.over] Status: New Submitter: Paolo Carlini Date: 2006-03-07
View all issues with New status.
Discussion:
log2 is missing from the list of "additional overloads" in TR1 8.16.4 [tr.c99.cmath.over] p1.
Hinnant: This is a TR1 issue only. It is fixed in the current (N2135) WD.
Proposed resolution:
Add log2 to the list of functions in TR1 8.16.4 [tr.c99.cmath.over] p1.
Section: 27.4.3 [fpos] Status: Open Submitter: Beman Dawes Date: 2006-04-12
View all other issues in [fpos].
View all issues with Open status.
Discussion:
There are two deficiencies related to file sizes:
The Dinkumware implementation of the Standard Library as shipped with the Microsoft compiler copes with these issues by:
fpos_t seekpos() const;
Because there are so many types relating to file positions and offsets (fpos_t, fpos, pos_type, off_type, streamoff, streamsize, streampos, wstreampos, and perhaps more), it is difficult to know if the Dinkumware extensions are sufficient. But they seem a useful starting place for discussions, and they do represent existing practice.
[ Kona (2007): We need a paper. It would be nice if someone proposed clarifications to the definitions of pos_type and off_type. Currently these definitions are horrible. Proposed Disposition: Open ]
Proposed resolution:
Section: 20.1.2 [allocator.requirements] Status: Open Submitter: Martin Sebor Date: 2006-06-14
View other active issues in [allocator.requirements].
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Duplicate of: 479
Discussion:
C++ Standard Library templates that take an allocator as an argument
are required to call the allocate() and
deallocate() members of the allocator object to obtain
storage. However, they do not appear to be required to call any other
allocator members such as construct(),
destroy(), address(), and
max_size(). This makes these allocator members less than
useful in portable programs.
It's unclear to me whether the absence of the requirement to use these allocator members is an unintentional omission or a deliberate choice. However, since the functions exist in the standard allocator and since they are required to be provided by any user-defined allocator I believe the standard ought to be clarified to explictly specify whether programs should or should not be able to rely on standard containers calling the functions.
I propose that all containers be required to make use of these functions.
[ Batavia: We support this resolution. Martin to provide wording. ]
[ pre-Oxford: Martin provided wording. ]
Proposed resolution:
Specifically, I propose to change 23.1 [container.requirements], p9 as follows:
-9- Copy constructors for all container types defined in this clause that are parametrized on
Allocatorcopyanthe allocator argument from their respective first parameters. All other constructors for these container types take anconstAllocator&argument (20.1.6), an allocator whosevalue_typeis the same as the container'svalue_type. A copy of this argumentisshall be used for any memory allocation and deallocation performed,by these constructors and by all member functions,during the lifetime of each container object. Allocation shall be performed "as if" by calling theallocate()member function on a copy of the allocator object of the appropriate type New Footnote), and deallocation "as if" by callingdeallocate()on a copy of the same allocator object of the corresponding type. A copy of this argument shall also be used to construct and destroy objects whose lifetime is managed by the container, including but not limited to those of the container'svalue_type, and to obtain their address. All objects residing in storage allocated by a container's allocator shall be constructed "as if" by calling theconstruct()member function on a copy of the allocator object of the appropriate type. The same objects shall be destroyed "as if" by callingdestroy()on a copy of the same allocator object of the same type. The address of such objects shall be obtained "as if" by calling theaddress()member function on a copy of the allocator object of the appropriate type. Finally, a copy of this argument shall be used by its container object to determine the maximum number of objects of the container'svalue_typethe container may store at the same time. The container member functionmax_size()obtains this number from the value returned by a call toget_allocator().max_size(). In all container types defined in this clause that are parametrized onAllocator, the memberget_allocator()returns a copy of theAllocatorobject used to construct the container.258)New Footnote: This type may be different from
Allocator: it may be derived fromAllocatorviaAllocator::rebind<U>::otherfor the appropriate typeU.
The proposed wording seems cumbersome but I couldn't think of a better
way to describe the requirement that containers use their
Allocator to manage only objects (regardless of their
type) that persist over their lifetimes and not, for example,
temporaries created on the stack. That is, containers shouldn't be
required to call Allocator::construct(Allocator::allocate(1),
elem) just to construct a temporary copy of an element, or
Allocator::destroy(Allocator::address(temp), 1) to
destroy temporaries.
[ Howard: This same paragraph will need some work to accommodate 431. ]
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
Section: 20.8.11.2 [uninitialized.copy] Status: Open Submitter: Martin Sebor Date: 2006-06-14
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Discussion:
The specialized algorithms [lib.specialized.algorithms] are specified as having the general effect of invoking the following expression:
new (static_cast<void*>(&*i))
typename iterator_traits<ForwardIterator>::value_type (x)
This expression is ill-formed when the type of the subexpression
&*i is some volatile-qualified T.
[ Batavia: Lack of support for proposed resolution but agree there is a defect. Howard to look at wording. Concern that move semantics properly expressed if iterator returns rvalue. ]
Proposed resolution:
In order to allow these algorithms to operate on volatile storage I propose to change the expression so as to make it well-formed even for pointers to volatile types. Specifically, I propose the following changes to clauses 20 and 24. Change 20.6.4.1, p1 to read:
Effects:
typedef typename iterator_traits<ForwardIterator>::pointer pointer;
typedef typename iterator_traits<ForwardIterator>::value_type value_type;
for (; first != last; ++result, ++first)
new (static_cast<void*>(const_cast<pointer>(&*result))
value_type (*first);
change 20.6.4.2, p1 to read
Effects:
typedef typename iterator_traits<ForwardIterator>::pointer pointer;
typedef typename iterator_traits<ForwardIterator>::value_type value_type;
for (; first != last; ++result, ++first)
new (static_cast<void*>(const_cast<pointer>(&*first))
value_type (*x);
and change 20.6.4.3, p1 to read
Effects:
typedef typename iterator_traits<ForwardIterator>::pointer pointer;
typedef typename iterator_traits<ForwardIterator>::value_type value_type;
for (; n--; ++first)
new (static_cast<void*>(const_cast<pointer>(&*first))
value_type (*x);
In addition, since there is no partial specialization for
iterator_traits<volatile T*> I propose to add one
to parallel such specialization for <const T*>. Specifically, I
propose to add the following text to the end of 24.3.1, p3:
and for pointers to volatile as
namespace std {
template<class T> struct iterator_traits<volatile T*> {
typedef ptrdiff_t difference_type;
typedef T value_type;
typedef volatile T* pointer;
typedef volatile T& reference;
typedef random_access_iterator_tag iterator_category;
};
}
Note that the change to iterator_traits isn't necessary
in order to implement the specialized algorithms in a way that allows
them to operate on volatile strorage. It is only necesassary in order
to specify their effects in terms of iterator_traits as
is done here. Implementations can (and some do) achieve the same
effect by means of function template overloading.
Section: 22.2 [locale.categories] Status: Open Submitter: Martin Sebor, Paolo Carlini Date: 2006-06-22
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Discussion:
Section 22.2, paragraph 2 requires facet get() members
that take an ios_base::iostate& argument,
err, to ignore the (initial) value of the
argument, but to set it to ios_base::failbit in case of a
parse error.
We believe there are a few minor problems with this blanket
requirement in conjunction with the wording specific to each
get() member function.
First, besides get() there are other member functions
with a slightly different name (for example,
get_date()). It's not completely clear that the intent of
the paragraph is to include those as well, and at least one
implementation has interpreted the requirement literally.
Second, the requirement to "set the argument to
ios_base::failbit suggests that the functions are not
permitted to set it to any other value (such as
ios_base::eofbit, or even ios_base::eofbit |
ios_base::failbit).
However, 22.2.2.1.2, p5 (Stage 3 of num_get parsing) and
p6 (bool parsing) specifies that the do_get
functions perform err |= ios_base::eofbit, which
contradicts the earlier requirement to ignore err's initial
value.
22.2.6.1.2, p1 (the Effects clause of the money_get
facet's do_get member functions) also specifies that
err's initial value be used to compute the final
value by ORing it with either ios_base::failbit or
withios_base::eofbit | ios_base::failbit.
Proposed resolution:
We believe the intent is for all facet member functions that take an
ios_base::iostate& argument to:
err argument,
err to ios_base::goodbit prior
to any further processing,
ios_base::eofbit, or
ios_base::failbit, or both in err, as
appropriate, in response to reaching the end-of-file or on parse
error, or both.
To that effect we propose to change 22.2, p2 as follows:
The put() members make no provision for error
reporting. (Any failures of the OutputIterator argument must be
extracted from the returned iterator.) Unless otherwise
specified, the get() members that
take an ios_base::iostate& argument whose value
they ignore, but set to ios_base::failbit in case of a parse
error., err, start by evaluating
err = ios_base::goodbit, and may subsequently set
err to either ios_base::eofbit, or
ios_base::failbit, or ios_base::eofbit |
ios_base::failbit in response to reaching the end-of-file or in
case of a parse error, or both, respectively.
[ Kona (2007): We need to change the proposed wording to clarify that the phrase "the get members" actually denotes get(), get_date(), etc. Proposed Disposition: Open ]
Section: 23.2.1 [array] Status: Open Submitter: Gennaro Prota Date: 2006-07-18
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Discussion:
The wording used for section 23.2.1 [lib.array] seems to be subtly ambiguous about zero sized arrays (N==0). Specifically:
* "An instance of array<T, N> stores N elements of type T, so that [...]"
Does this imply that a zero sized array object stores 0 elements, i.e. that it cannot store any element of type T? The next point clarifies the rationale behind this question, basically how to implement begin() and end():
* 23.2.1.5 [lib.array.zero], p2: "In the case that N == 0, begin() == end() == unique value."
What does "unique" mean in this context? Let's consider the following possible implementations, all relying on a partial specialization:
a)
template< typename T >
class array< T, 0 > {
....
iterator begin()
{ return iterator( reinterpret_cast< T * >( this ) ); }
....
};
This has been used in boost, probably intending that the return value had to be unique to the specific array object and that array couldn't store any T. Note that, besides relying on a reinterpret_cast, has (more than potential) alignment problems.
b)
template< typename T >
class array< T, 0 > {
T t;
iterator begin()
{ return iterator( &t ); }
....
};
This provides a value which is unique to the object and to the type of the array, but requires storing a T. Also, it would allow the user to mistakenly provide an initializer list with one element.
A slight variant could be returning *the* null pointer of type T
return static_cast<T*>(0);
In this case the value would be unique to the type array<T, 0> but not to the objects (all objects of type array<T, 0> with the same value for T would yield the same pointer value).
Furthermore this is inconsistent with what the standard requires from allocation functions (see library issue 9).
c) same as above but with t being a static data member; again, the value would be unique to the type, not to the object.
d) to avoid storing a T *directly* while disallowing the possibility to use a one-element initializer list a non-aggregate nested class could be defined
struct holder { holder() {} T t; } h;
and then begin be defined as
iterator begin() { return &h.t; }
But then, it's arguable whether the array stores a T or not. Indirectly it does.
-----------------------------------------------------
Now, on different issues:
* what's the effect of calling assign(T&) on a zero-sized array? There seems to be only mention of front() and back(), in 23.2.1 [lib.array] p4 (I would also suggest to move that bullet to section 23.2.1.5 [lib.array.zero], for locality of reference)
* (minor) the opening paragraph of 23.2.1 [lib.array] wording is a bit inconsistent with that of other sequences: that's not a problem in itself, but compare it for instance with "A vector is a kind of sequence that supports random access iterators"; though the intent is obvious one might argue that the wording used for arrays doesn't tell what an array is, and relies on the reader to infer that it is what the <array> header defines.
* it would be desiderable to have a static const data member of type std::size_t, with value N, for usage as integral constant expression
* section 23.1 [lib.container.requirements] seem not to consider fixed-size containers at all, as it says: "[containers] control allocation and deallocation of these objects [the contained objects] through constructors, destructors, *insert and erase* operations"
* max_size() isn't specified: the result is obvious but, technically, it relies on table 80: "size() of the largest possible container" which, again, doesn't seem to consider fixed size containers
Proposed resolution:
[ Kona (2007): requirements on zero sized tr1::arrays and other details Issue 617: std::array is a sequence that doesn't satisfy the sequence requirements? Alisdair will prepare a paper. Proposed Disposition: Open ]
Section: TRDecimal 3.2 [trdec.types.types] Status: Open Submitter: Daveed Vandevoorde Date: 2006-04-05
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Discussion:
In a private email, Daveed writes:
I am not familiar with the C TR, but my guess is that the class type approach still won't match a built-in type approach because the notion of "promotion" cannot be emulated by user-defined types.
Here is an example:
struct S {
S(_Decimal32 const&); // Converting constructor
};
void f(S);
void f(_Decimal64);
void g(_Decimal32 d) {
f(d);
}
If _Decimal32 is a built-in type, the call f(d) will likely resolve to f(_Decimal64) because that requires only a promotion, whereas f(S) requires a user-defined conversion.
If _Decimal32 is a class type, I think the call f(d) will be ambiguous because both the conversion to _Decimal64 and the conversion to S will be user-defined conversions with neither better than the other.
Robert comments:
In general, a library of arithmetic types cannot exactly emulate the behavior of the intrinsic numeric types. There are several ways to tell whether an implementation of the decimal types uses compiler intrinisics or a library. For example:
_Decimal32 d1;
d1.operator+=(5); // If d1 is a builtin type, this won't compile.
In preparing the decimal TR, we have three options:
We decided as a group to pursue option #3, but that approach implies that implementations may not agree on the semantics of certain use cases (first example, above), or on whether certain other cases are well-formed (second example). Another potentially important problem is that, under the present definition of POD, the decimal classes are not POD types, but builtins will be.
Note that neither example above implies any problems with respect to C-to-C++ compatibility, since neither example can be expressed in C.
Proposed resolution:
Section: TRDecimal 3.2 [trdec.types.types] Status: Open Submitter: Martin Sebor Date: 2006-06-15
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Discussion:
In c++std-lib-17205, Martin writes:
...was it a deliberate design choice to make narrowing assignments ill-formed while permitting narrowing compound assignments? For instance:
decimal32 d32;
decimal64 d64;
d32 = 64; // error
d32 += 64; // okay
In c++std-lib-17229, Robert responds:
It is a vestige of an old idea that I forgot to remove from the paper. Narrowing assignments should be permitted. The bug is that the converting constructors that cause narrowing should not be explicit. Thanks for pointing this out.
Proposed resolution:
1. In "3.2.2 Class decimal32" synopsis, remove the explicit specifier from the narrowing conversions:
// 3.2.2.2 conversion from floating-point type:
explicit decimal32(decimal64 d64);
explicit decimal32(decimal128 d128);
2. Do the same thing in "3.2.2.2. Conversion from floating-point type."
3. In "3.2.3 Class decimal64" synopsis, remove the explicit specifier from the narrowing conversion:
// 3.2.3.2 conversion from floating-point type:
explicit decimal64(decimal128 d128);
4. Do the same thing in "3.2.3.2. Conversion from floating-point type."
[ Redmond: We prefer explicit conversions for narrowing and implicit for widening. ]
Section: 21.3 [basic.string] Status: Open Submitter: Bo Persson Date: 2006-12-05
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Discussion:
This is based on N2134, where 21.3.1/2 states: "... The Allocator object used shall be a copy of the Allocator object passed to the basic_string object's constructor or, if the constructor does not take an Allocator argument, a copy of a default-constructed Allocator object."
Section 21.3.2/1 lists two constructors:
basic_string(const basic_string<charT,traits,Allocator>& str );
basic_string(const basic_string<charT,traits,Allocator>& str ,
size_type pos , size_type n = npos,
const Allocator& a = Allocator());
and then says "In the first form, the Allocator value used is copied from str.get_allocator().", which isn't an option according to 21.3.1.
[ Batavia: We need blanket statement to the effect of: ]
[ Review constructors and functions that return a string; make sure we follow these rules (substr, operator+, etc.). Howard to supply wording. ]
[
Bo adds: The new container constructor which takes only a size_type is not
consistent with 23.1 [container.requirements], p9 which says in part:
All other constructors for these container types take an
Allocator& argument (20.1.2), an allocator whose value type
is the same as the container's value type. A copy of this argument is
used for any memory allocation performed, by these constructors and by
all member functions, during the lifetime of each container object.
]
[ post Bellevue: We re-confirm that the issue is real. Pablo will provide wording. ]
Proposed resolution:
Section: 23.2.1 [array] Status: Open Submitter: Bo Persson Date: 2006-12-30
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Discussion:
The <array> header is given under 23.2 [sequences]. 23.2.1 [array]/paragraph 3 says:
"Unless otherwise specified, all array operations are as described in 23.1 [container.requirements]".
However, array isn't mentioned at all in section 23.1 [container.requirements]. In particular, Table 82 "Sequence requirements" lists several operations (insert, erase, clear) that std::array does not have in 23.2.1 [array].
Also, Table 83 "Optional sequence operations" lists several operations that std::array does have, but array isn't mentioned.
Proposed resolution:
Section: 17 [library] Status: Open Submitter: Martin Sebor Date: 2007-01-20
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Discussion:
Many member functions of basic_string are overloaded,
with some of the overloads taking a string argument,
others value_type*, others size_type, and
others still iterators. Often, the requirements on one of
the overloads are expressed in the form of Effects,
Throws, and in the Working Paper
(N2134)
also Remark clauses, while those on the rest of the overloads
via a reference to this overload and using a Returns clause.
The difference between the two forms of specification is that per 17.5.2.4 [structure.specifications], p3, an Effects clause specifies "actions performed by the functions," i.e., its observable effects, while a Returns clause is "a description of the return value(s) of a function" that does not impose any requirements on the function's observable effects.
Since only Notes are explicitly defined to be informative and all other paragraphs are explicitly defined to be normative, like Effects and Returns, the new Remark clauses also impose normative requirements.
So by this strict reading of the standard there are some member functions of
basic_string that are required to throw an
exception under some conditions or use specific traits members while
many other otherwise equivalent overloads, while obliged to return the
same values, aren't required to follow the exact same requirements
with regards to the observable effects.
Here's an example of this problem that was precipitated by the change from informative Notes to normative Remarks (presumably made to address 424):
In the Working Paper,
find(string, size_type) contains a
Remark clause (which is just a Note in the current
standard) requiring it to use traits::eq().
find(const charT *s, size_type pos) is specified to
return find(string(s), pos) by a Returns clause
and so it is not required to use traits::eq(). However,
the Working Paper has replaced the original informative Note
about the function using traits::length() with a
normative requirement in the form of a Remark. Calling
traits::length() may be suboptimal, for example when the
argument is a very long array whose initial substring doesn't appear
anywhere in *this.
Here's another similar example, one that existed even prior to the introduction of Remarks:
insert(size_type pos, string, size_type, size_type) is
required to throw out_of_range if pos >
size().
insert(size_type pos, string str) is specified to return
insert(pos, str, 0, npos) by a Returns clause and
so its effects when pos > size() are strictly speaking
unspecified.
I believe a careful review of the current Effects and Returns clauses is needed in order to identify all such problematic cases. In addition, a review of the Working Paper should be done to make sure that the newly introduced normative Remark clauses do not impose any undesirable normative requirements in place of the original informative Notes.
[ Batavia: Alan and Pete to work. ]
[ Bellevue: Marked as NAD Editorial. ]
[ Post-Sophia Antipolis: Martin indicates there is still work to be done on this issue. Reopened. ]
Proposed resolution:
Section: 26.5.2.1 [valarray.cons] Status: Open Submitter: Martin Sebor Date: 2007-01-28
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Discussion:
Section 26.1 [numeric.requirements], p1 suggests that a
valarray specialization on a type T that
satisfies the requirements enumerated in the paragraph is itself a
valid type on which valarray may be instantiated
(Footnote 269 makes this clear). I.e.,
valarray<valarray<T> > is valid as long as
T is valid. However, since implementations of
valarray are permitted to initialize storage allocated by
the class by invoking the default ctor of T followed by
the copy assignment operator, such implementations of
valarray wouldn't work with (perhaps user-defined)
specializations of valarray whose assignment operator had
undefined behavior when the size of its argument didn't match the size
of *this. By "wouldn't work" I mean that it would
be impossible to resize such an array of arrays by calling the
resize() member function on it if the function used the
copy assignment operator after constructing all elements using the
default ctor (e.g., by invoking new value_type[N]) to
obtain default-initialized storage) as it's permitted to do.
Stated more generally, the problem is that
valarray<valarray<T> >::resize(size_t) isn't
required or guaranteed to have well-defined semantics for every type
T that satisfies all requirements in
26.1 [numeric.requirements].
I believe this problem was introduced by the adoption of the
resolution outlined in N0857,
Assignment of valarrays, from 1996. The copy assignment
operator of the original numerical array classes proposed in N0280,
as well as the one proposed in N0308
(both from 1993), had well-defined semantics for arrays of unequal
size (the latter explicitly only when *this was empty;
assignment of non empty arrays of unequal size was a runtime error).
The justification for the change given in N0857 was the "loss of performance [deemed] only significant for very simple operations on small arrays or for architectures with very few registers."
Since tiny arrays on a limited subset of hardware architectures are
likely to be an exceedingly rare case (despite the continued
popularity of x86) I propose to revert the resolution and make the
behavior of all valarray assignment operators
well-defined even for non-conformal arrays (i.e., arrays of unequal
size). I have implemented this change and measured no significant
degradation in performance in the common case (non-empty arrays of
equal size). I have measured a 50% (and in some cases even greater)
speedup in the case of assignments to empty arrays versus calling
resize() first followed by an invocation of the copy
assignment operator.
[ Bellevue: ]
If no proposed wording by June meeting, this issue should be closed NAD.
Proposed resolution:
Change 26.5.2.2 [valarray.assign], p1 as follows:
valarray<T>& operator=(const valarray<T>& x);-1- Each element of the
*thisarray is assigned the value of the corresponding element of the argument array.The resulting behavior is undefined ifWhen the length of the argument array is not equal to the length of the *this array.resizes*thisto make the two arrays the same length, as if by callingresize(x.size()), before performing the assignment.
And add a new paragraph just below paragraph 1 with the following text:
-2- Postcondition:
size() == x.size().
Also add the following paragraph to 26.5.2.2 [valarray.assign], immediately after p4:
-?- When the length,
Nof the array referred to by the argument is not equal to the length of*this, the operator resizes*thisto make the two arrays the same length, as if by callingresize(N), before performing the assignment.
[ pre-Sophia Antipolis, Martin adds the following compromise wording, but prefers the original proposed resolution: ]
Change 26.5.2.2 [valarray.assign], p1 as follows:
-1- Requires: size() == 0 || size() == x.size().
-2- Effects: If size() == 0 calls x.resize(x.size()) first. Each element of the *this array is assigned the value of the corresponding element of the argument array.
-3- Postcondition: size() == x.size().
Add the following paragraph to 26.5.2.2 [valarray.assign], immediately after p4:
-?- When size() == 0 and the length, N of the array referred to by the argument is not equal to the length of *this, the operator resizes *this to make the two arrays the same length, as if by calling resize(N), before performing the assignment. Otherwise, when size() > 0 and size() != N, the behavior is undefined.
[ Kona (2007): Gaby to propose wording for an alternative resolution in which you can assign to a valarray of size 0, but not to any other valarray whose size is unequal to the right hand side of the assignment. ]
Section: 23.1 [container.requirements] Status: Open Submitter: Lionel B Date: 2007-02-01
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Discussion:
A recent news group discussion:
Anyone know if the Standard has anything to say about the time complexity of size() for std::set? I need to access a set's size (/not/ to know if it is empty!) heavily during an algorithm and was thus wondering whether I'd be better off tracking the size "manually" or whether that'd be pointless.
That would be pointless. size() is O(1).
Nit: the standard says "should" have constant time. Implementations may take license to do worse. I know that some do this for std::list<> as a part of some trade-off with other operation.
I was aware of that, hence my reluctance to use size() for std::set.
However, this reason would not apply to std::set<> as far as I can see.
Ok, I guess the only option is to try it and see...
If I have any recommendation to the C++ Standards Committee it is that implementations must (not "should"!) document clearly[1], where known, the time complexity of *all* container access operations.
[1] In my case (gcc 4.1.1) I can't swear that the time complexity of size() for std::set is not documented... but if it is it's certainly well hidden away.
Proposed resolution:
[ Kona (2007): This issue affects all the containers. We'd love to see a paper dealing with the broad issue. We think that the complexity of the size() member of every container -- except possibly list -- should be O(1). Alan has volunteered to provide wording. ]
[ Bellevue: ]
Mandating O(1) size will not fly, too many implementations would be invalidated. Alan to provide wording that toughens wording, but that does not absolutely mandate O(1).
Section: 20.1.2 [allocator.requirements] Status: Open Submitter: Howard Hinnant Date: 2007-02-08
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Discussion:
The table of allocator requirements in 20.1.2 [allocator.requirements] describes allocator::address as:
a.address(r) a.address(s)
where r and s are described as:
a value of type X::reference obtained by the expression *p.
and p is
a value of type X::pointer, obtained by calling a1.allocate, where a1 == a
This all implies that to get the address of some value of type T that value must have been allocated by this allocator or a copy of it.
However sometimes container code needs to compare the address of an external value of type T with an internal value. For example list::remove(const T& t) may want to compare the address of the external value t with that of a value stored within the list. Similarly vector or deque insert may want to make similar comparisons (to check for self-referencing calls).
Mandating that allocator::address can only be called for values which the allocator allocated seems overly restrictive.
[ post San Francisco: ]
Pablo recommends NAD Editorial, solved by N2768.
Proposed resolution:
Change 20.1.2 [allocator.requirements]:
r : a value of type X::reference
obtained by the expression *p.s : a value of type X::const_reference
obtained by the expression *q or by conversion from a value r.
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
[ Kona (2007): This issue is section 8 of N2387. There was some discussion of it but no resolution to this issue was recorded. Moved to Open. ]
Section: 20.7.16.2 [func.wrap.func] Status: Open Submitter: Bo Persson Date: 2007-02-25
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Discussion:
20.7.16.2 [func.wrap.func]
The note in paragraph 2 refers to 'undefined void operators', while the section declares a pair of operators returning bool.
[ Post-Sophia Antipolis: ]
Changed from Pending WP to Open. This issue was voted to WP at the same time the operators were changed from private to deleted. The two issues stepped on each other. What do we want the return type of these deleted functions to be?
Proposed resolution:
Change 20.7.16.2 [func.wrap.func]
... private: // 20.7.16.2 [func.wrap.func], undefined operators: template<class Function2>boolvoid operator==(const function<Function2>&); template<class Function2>boolvoid operator!=(const function<Function2>&); };
Change 20.7.16.2 [func.wrap.func]
template<class Function2>boolvoid operator==(const function<Function2>&); template<class Function2>boolvoid operator!=(const function<Function2>&);
Section: 24.5.3 [istreambuf.iterator] Status: Open Submitter: Niels Dekker Date: 2007-03-25
View all other issues in [istreambuf.iterator].
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Discussion:
Greg Herlihy has clearly demonstrated that a user defined input iterator should have an operator->(), even if its value type is a built-in type (comp.std.c++, "Re: Should any iterator have an operator->() in C++0x?", March 2007). And as Howard Hinnant remarked in the same thread that the input iterator istreambuf_iterator doesn't have one, this must be a defect!
Based on Greg's example, the following code demonstrates the issue:
#include <iostream>
#include <fstream>
#include <streambuf>
typedef char C;
int main ()
{
std::ifstream s("filename", std::ios::in);
std::istreambuf_iterator<char> i(s);
(*i).~C(); // This is well-formed...
i->~C(); // ... so this should be supported!
}
Of course, operator-> is also needed when the value_type of istreambuf_iterator is a class.
The operator-> could be implemented in various ways. For instance, by storing the current value inside the iterator, and returning its address. Or by returning a proxy, like operator_arrow_proxy, from http://www.boost.org/boost/iterator/iterator_facade.hpp
I hope that the resolution of this issue will contribute to getting a clear and consistent definition of iterator concepts.
Proposed resolution:
Add to the synopsis in 24.5.3 [istreambuf.iterator]:
charT operator*() const; pointer operator->() const; istreambuf_iterator<charT,traits>& operator++();
Change 24.5.3 [istreambuf.iterator], p1:
The class template istreambuf_iterator reads successive characters from the streambuf for which it was constructed. operator* provides access to the current input character, if any. operator-> may return a proxy. Each time operator++ is evaluated, the iterator advances to the next input character. If the end of stream is reached (streambuf_type::sgetc() returns traits::eof()), the iterator becomes equal to the end of stream iterator value. The default constructor istreambuf_iterator() and the constructor istreambuf_iterator(0) both construct an end of stream iterator object suitable for use as an end-of-range.
[ Kona (2007): The proposed resolution is inconsistent because the return type of istreambuf_iterator::operator->() is specified to be pointer, but the proposed text also states that "operator-> may return a proxy." ]
[ Niels Dekker (mailed to Howard Hinnant): ]
The proposed resolution does not seem inconsistent to me. istreambuf_iterator::operator->() should have istreambuf_iterator::pointer as return type, and this return type may in fact be a proxy.
AFAIK, the resolution of 445 ("iterator_traits::reference unspecified for some iterator categories") implies that for any iterator class Iter, the return type of operator->() is Iter::pointer, by definition. I don't think Iter::pointer needs to be a raw pointer.
Still I wouldn't mind if the text "operator-> may return a proxy" would be removed from the resolution. I think it's up to the library implementation, how to implement istreambuf_iterator::operator->(). As longs as it behaves as expected: i->m should have the same effect as (*i).m. Even for an explicit destructor call, i->~C(). The main issue is just: istreambuf_iterator should have an operator->()!
Section: 22.2.6.1.2 [locale.money.get.virtuals] Status: Open Submitter: Thomas Plum Date: 2007-04-16
View other active issues in [locale.money.get.virtuals].
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Discussion:
22.2.6.1.2 [locale.money.get.virtuals], para 1 says:
The result is returned as an integral value stored in units or as a sequence of digits possibly preceded by a minus sign (as produced by ct.widen(c) where c is '-' or in the range from '0' through '9', inclusive) stored in digits.
The following objection has been raised:
Some implementations interpret this to mean that a facet derived from ctype<wchar_t> can provide its own member do_widen(char) which produces e.g. L'@' for the "widened" minus sign, and that the '@' symbol will appear in the resulting sequence of digits. Other implementations have assumed that one or more places in the standard permit the implementation to "hard-wire" L'-' as the "widened" minus sign. Are both interpretations permissible, or only one?
[Plum ref _222612Y14]
Furthermore: if ct.widen('9') produces L'X' (a non-digit), does a parse fail if a '9' appears in the subject string? [Plum ref _22263Y33]
[ Kona (2007): Bill and Dietmar to provide proposed wording. ]
[ post Bellevue: Bill adds: ]
The Standard is clear that the minus sign stored in digits is ct.widen('-'). The subject string must contain characters c in the set [-0123456789] which are translated by ct.widen(c) calls before being stored in digits; the widened characters are not relevant to the parsing of the subject string.
Proposed resolution:
Section: 22.2.6.1.2 [locale.money.get.virtuals] Status: Open Submitter: Thomas Plum Date: 2007-04-16
View other active issues in [locale.money.get.virtuals].
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Discussion:
22.2.6.1.2 [locale.money.get.virtuals], para 3 says:
If pos or neg is empty, the sign component is optional, and if no sign is detected, the result is given the sign that corresponds to the source of the empty string.
The following objection has been raised:
A negative_sign of "" means "there is no way to write a negative sign" not "any null sequence is a negative sign, so it's always there when you look for it".
[Plum ref _222612Y32]
[ Kona (2007): Bill to provide proposed wording and interpretation of existing wording. ]
Proposed resolution:
Section: 22.2.6.1.2 [locale.money.get.virtuals] Status: Open Submitter: Thomas Plum Date: 2007-04-16
View other active issues in [locale.money.get.virtuals].
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Discussion:
22.2.6.1.2 [locale.money.get.virtuals], para 3 sentence 4 says:
If the first character of pos is equal to the first character of neg, or if both strings are empty, the result is given a positive sign.
One interpretation is that an input sequence must match either the positive pattern or the negative pattern, and then in either event it is interpreted as positive. The following objections has been raised:
The input can successfully match only a positive sign, so the negative pattern is an unsuccessful match.
[Plum ref _222612Y34, 222612Y51b]
[ Bill to provide proposed wording and interpretation of existing wording. ]
Proposed resolution:
Section: 22.2.2.2.2 [facet.num.put.virtuals] Status: Open Submitter: John Salmon Date: 2007-04-20
View all other issues in [facet.num.put.virtuals].
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Discussion:
I am trying to understand how TR1 supports hex float (%a) output.
As far as I can tell, it does so via the following:
8.15 Additions to header <locale> [tr.c99.locale]
In subclause 22.2.2.2.2 [facet.num.put.virtuals], Table 58 Floating-point conversions, after the line: floatfield == ios_base::scientific %E
add the two lines:
floatfield == ios_base::fixed | ios_base::scientific && !uppercase %a floatfield == ios_base::fixed | ios_base::scientific %A 2
[Note: The additional requirements on print and scan functions, later in this clause, ensure that the print functions generate hexadecimal floating-point fields with a %a or %A conversion specifier, and that the scan functions match hexadecimal floating-point fields with a %g conversion specifier. end note]
Following the thread, in 22.2.2.2.2 [facet.num.put.virtuals], we find:
For conversion from a floating-point type, if (flags & fixed) != 0 or if str.precision() > 0, then str.precision() is specified in the conversion specification.
This would seem to imply that when floatfield == fixed|scientific, the precision of the conversion specifier is to be taken from str.precision(). Is this really what's intended? I sincerely hope that I'm either missing something or this is an oversight. Please tell me that the committee did not intend to mandate that hex floats (and doubles) should by default be printed as if by %.6a.
[ Howard: I think the fundamental issue we overlooked was that with %f, %e, %g, the default precision was always 6. With %a the default precision is not 6, it is infinity. So for the first time, we need to distinguish between the default value of precision, and the precision value 6. ]
Proposed resolution:
[ Kona (2007): Robert volunteers to propose wording. ]
Section: 20.7.5.1 [refwrap.const] Status: Open Submitter: Peter Dimov Date: 2007-05-10
View all other issues in [refwrap.const].
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Discussion:
A reference_wrapper can be constructed from an rvalue, either by using the constructor, or via cref (and ref in some corner cases). This leads to a dangling reference being stored into the reference_wrapper object. Now that we have a mechanism to detect an rvalue, we can fix them to disallow this source of undefined behavior.
Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.
Proposed resolution:
In 20.7 [function.objects], add the following two signatures to the synopsis:
template <class T> void ref(const T&& t) = delete; template <class T> void cref(const T&& t) = delete;
[ N2292 addresses the first part of the resolution but not the second. ]
[ Bellevue: Doug noticed problems with the current wording. ]
[ post Bellevue: Howard and Peter provided revised wording. ]
[ This resolution depends on a "favorable" resolution of CWG 606: that is, the "special deduction rule" is disabled with the const T&& pattern. ]
istream::operator>>(int&) brokenSection: 27.6.1.2.2 [istream.formatted.arithmetic] Status: New Submitter: Martin Sebor Date: 2007-06-23
View all other issues in [istream.formatted.arithmetic].
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Discussion:
From message c++std-lib-17897:
The code shown in 27.6.1.2.2 [istream.formatted.arithmetic] as the "as if"
implementation of the two arithmetic extractors that don't have a
corresponding num_get interface (i.e., the
short and int overloads) is subtly buggy in
how it deals with EOF, overflow, and other similar
conditions (in addition to containing a few typos).
One problem is that if num_get::get() reaches the EOF
after reading in an otherwise valid value that exceeds the limits of
the narrower type (but not LONG_MIN or
LONG_MAX), it will set err to
eofbit. Because of the if condition testing for
(err == 0), the extractor won't set
failbit (and presumably, return a bogus value to the
caller).
Another problem with the code is that it never actually sets the
argument to the extracted value. It can't happen after the call to
setstate() since the function may throw, so we need to
show when and how it's done (we can't just punt as say: "it happens
afterwards"). However, it turns out that showing how it's done isn't
quite so easy since the argument is normally left unchanged by the
facet on error except when the error is due to a misplaced thousands
separator, which causes failbit to be set but doesn't
prevent the facet from storing the value.
Proposed resolution:
Section: TR1 5.2.1.1 [tr.num.sf.Lnm] Status: New Submitter: Christopher Crawford Date: 2007-06-30
View all issues with New status.
Discussion:
I see that the definition the associated Laguerre polynomials TR1 5.2.1.1 [tr.num.sf.Lnm] has been corrected since N1687. However, the draft standard only specifies ranks of integer value m, while the associated Laguerre polynomials are actually valid for real values of m > -1. In the case of non-integer values of m, the definition Ln(m) = (1/n!)exx-m (d/dx)n (e-xxm+n) must be used, which also holds for integer values of m. See Abramowitz & Stegun, 22.11.6 for the general case, and 22.5.16-17 for the integer case. In fact fractional values are most commonly used in physics, for example to m = +/- 1/2 to describe the harmonic oscillator in 1 dimension, and 1/2, 3/2, 5/2, ... in 3 dimensions.
If I am correct, the calculation of the more general case is no more difficult, and is in fact the function implemented in the GNU Scientific Library. I would urge you to consider upgrading the standard, either adding extra functions for real m or switching the current ones to double.
Proposed resolution:
Section: TR1 5.2.1.2 [tr.num.sf.Plm] Status: New Submitter: Christopher Crawford Date: 2007-06-30
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Discussion:
One other small thing, in TR1 5.2.1.2 [tr.num.sf.Plm], the restriction should be |x| <= 1, not x >= 0.
Proposed resolution:
Section: 22 [localization] Status: Open Submitter: Peter Dimov Date: 2007-07-28
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Discussion:
The POSIX "Extended API Set Part 4,"
introduces extensions to the C locale mechanism that allow multiple concurrent locales to be used in the same application by introducing a type locale_t that is very similar to std::locale, and a number of _l functions that make use of it.
The global locale (set by setlocale) is now specified to be per- process. If a thread does not call uselocale, the global locale is in effect for that thread. It can install a per-thread locale by using uselocale.
There is also a nice querylocale mechanism by which one can obtain the name (such as "de_DE") for a specific facet, even for combined locales, with no std::locale equivalent.
std::locale should be harmonized with the new POSIX locale_t mechanism and provide equivalents for uselocale and querylocale.
[ Kona (2007): Bill and Nick to provide wording. ]
[ San Francisco: Bill and Nick still intend to provide wording, but this is a part of the task to be addressed by the group that will look into issue 860. ]
Proposed resolution:
Section: 20.8.13.2.5 [util.smartptr.shared.obs] Status: Open Submitter: Peter Dimov Date: 2007-08-24
View all other issues in [util.smartptr.shared.obs].
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Discussion:
A discussion on comp.std.c++ has identified a contradiction in the shared_ptr specification. The note:
[ Note: this constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer. -end note ]
after the aliasing constructor
template<class Y> shared_ptr(shared_ptr<Y> const& r, T *p);
reflects the intent of N2351 to, well, allow the creation of an empty shared_ptr with a non-NULL stored pointer.
This is contradicted by the second sentence in the Returns clause of 20.8.13.2.5 [util.smartptr.shared.obs]:
T* get() const;Returns: the stored pointer. Returns a null pointer if *this is empty.
[ Bellevue: ]
Adopt option 1 and move to review, not ready.
There was a lot of confusion about what an empty shared_ptr is (the term isn't defined anywhere), and whether we have a good mental model for how one behaves. We think it might be possible to deduce what the definition should be, but the words just aren't there. We need to open an issue on the use of this undefined term. (The resolution of that issue might affect the resolution of issue 711.)
The LWG is getting more uncomfortable with the aliasing proposal (N2351) now that we realize some of its implications, and we need to keep an eye on it, but there isn't support for removing this feature at this time.
[ Sophia Antipolis: ]
We heard from Peter Dimov, who explained his reason for preferring solution 1.
Because it doesn't seem to add anything. It simply makes the behavior for p = 0 undefined. For programmers who don't create empty pointers with p = 0, there is no difference. Those who do insist on creating them presumably have a good reason, and it costs nothing for us to define the behavior in this case.
The aliasing constructor is sharp enough as it is, so "protecting" users doesn't make much sense in this particular case.
> Do you have a use case for r being empty and r being non-null?
I have received a few requests for it from "performance-conscious" people (you should be familiar with this mindset) who don't like the overhead of allocating and maintaining a control block when a null deleter is used to approximate a raw pointer. It is obviously an "at your own risk", low-level feature; essentially a raw pointer behind a shared_ptr facade.
We could not agree upon a resolution to the issue; some of us thought that Peter's description above is supporting an undesirable behavior.
Proposed resolution:
In keeping the N2351 spirit and obviously my preference, change 20.8.13.2.5 [util.smartptr.shared.obs]:
T* get() const;Returns: the stored pointer.
Returns a null pointer if *this is empty.
Alternative proposed resolution: (I won't be happy if we do this, but it's possible):
Change 20.8.13.2.1 [util.smartptr.shared.const]:
template<class Y> shared_ptr(shared_ptr<Y> const& r, T *p);Requires: If r is empty, p shall be 0.
[ Note: this constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer. -- end note ]
Section: 28.13 [re.grammar] Status: New Submitter: Stephan T. Lavavej Date: 2007-08-31
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Discussion:
TR1 7.13 [tr.re.grammar]/3 and C++0x WP 28.13 [re.grammar]/3 say:
The following productions within the ECMAScript grammar are modified as follows:
CharacterClass :: [ [lookahead ∉ {^}] ClassRanges ] [ ^ ClassRanges ]
This definition for CharacterClass appears to be exactly identical to that in ECMA-262.
Was an actual modification intended here and accidentally omitted, or was this production accidentally included?
Proposed resolution:
Remove this mention of the CharacterClass production.
CharacterClass :: [ [lookahead ∉ {^}] ClassRanges ] [ ^ ClassRanges ]
Section: 21.3 [basic.string] Status: Open Submitter: Bo Persson Date: 2007-08-18
View other active issues in [basic.string].
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Discussion:
Paragraph 21.3 [basic.string]/3 states:
The class template basic_string conforms to the requirements for a Sequence (23.1.1) and for a Reversible Container (23.1).
First of all, 23.1.3 [sequence.reqmts] is no longer "Sequence" but "Sequence container". Secondly, after the resent changes to containers (emplace, push_back, const_iterator parameters to insert and erase), basic_string is not even close to conform to the current requirements.
[ Bellevue: ]
- emplace, for example, may not make sense for strings. Is also likely suboptimal
- with concepts do we need to maintain string as sequence container?
- One approach might be to say something like: string is a sequence except it doesn't have these functions
General consensus is to suggest option 2.
- basic_string already has push_back
- const_iterator parameters to insert and erase should be added to basic_string
- this leaves emplace to handle -- we have the following options:
- option 1: add it to string even though it's optional
- option 2: make emplace optional to sequences (move from table 89 to 90)
- option 3: say string not sequence (the proposal),
- option 4: add an exception to basic string wording.
Proposed resolution:
Remove this sentence, in recognition of the fact that basic_string is not just a vector-light for literal types, but something quite different, a string abstraction in its own right.
Section: 20.6 [meta] Status: Open Submitter: Daniel Krügler Date: 2007-08-25
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Discussion:
Since the inclusion of constexpr in the standard draft N2369 we have a new type category "literal", which is defined in 3.9 [basic.types]/p.11:
-11- A type is a literal type if it is:
- a scalar type; or
a class type (clause 9) with
- a trivial copy constructor,
- a trivial destructor,
- at least one constexpr constructor other than the copy constructor,
- no virtual base classes, and
- all non-static data members and base classes of literal types; or
- an array of literal type.
I strongly suggest that the standard provides a type traits for literal types in 20.6.4.3 [meta.unary.prop] for several reasons:
The special problem of reason (c) is that I don't see currently a way to portably test the condition for literal class types:
- at least one constexpr constructor other than the copy constructor,
Here follows a simply example to demonstrate it's usefulness:
template <typename T>
constexpr typename std::enable_if<std::is_literal<T>::value, T>::type
abs(T x) {
return x < T() ? -x : x;
}
template <typename T>
typename std::enable_if<!std::is_literal<T>::value, T>::type
abs(const T& x) {
return x < T() ? -x : x;
}
Here we have the possibility to provide a general abs function template that can be used in ICE's if it's argument is a literal type which's value is a constant expression, otherwise we have an optimized version for arguments which are expensive to copy and therefore need the usage of arguments of reference type (instead of const T& we could decide to use T&&, but that is another issue).
[ Alisdair is considering preparing a paper listing a number of missing type traits, and feels that it might be useful to handle them all together rather than piecemeal. This would affect issue 719 and 750. These two issues should move to OPEN pending AM paper on type traits. ]
Proposed resolution:
In 20.6.2 [meta.type.synop] in the group "type properties", just below the line
template <class T> struct is_pod;
add a new one:
template <class T> struct is_literal;
In 20.6.4.3 [meta.unary.prop], table Type Property Predicates, just below the line for the is_pod property add a new line:
| Template | Condition | Preconditions |
|---|---|---|
| template <class T> struct is_literal; | T is a literal type (3.9) | T shall be a complete type, an array of unknown bound, or (possibly cv-qualified) void. |
Section: 22.1.3.2.2 [conversions.string] Status: Open Submitter: Bo Persson Date: 2007-08-27
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Discussion:
Paragraph 3 says that the Codecvt template parameter shall meet the requirements of std::codecvt, even though std::codecvt itself cannot be used (because of a protected destructor).
How are we going to explain this code to beginning programmers?
template<class I, class E, class S>
struct codecvt : std::codecvt<I, E, S>
{
~codecvt()
{ }
};
void main()
{
std::wstring_convert<codecvt<wchar_t, char, std::mbstate_t> > compiles_ok;
std::wstring_convert<std::codecvt<wchar_t, char, std::mbstate_t> > not_ok;
}
[ San Francisco: ]
Bill will propose a resolution.
Proposed resolution:
Section: 28.8 [re.regex] Status: Open Submitter: Daniel Krügler Date: 2007-08-29
View all other issues in [re.regex].
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Discussion:
According to the current state of the standard draft, the class template basic_regex, as described in 28.8 [re.regex]/3, is neither MoveConstructible nor MoveAssignable. IMO it should be, because typical regex state machines tend to have a rather large data quantum and I have seen several use cases, where a factory function returns regex values, which would take advantage of moveabilities.
[ Sophia Antipolis: ]
Needs wording for the semantics, the idea is agreed upon.
Proposed resolution:
In the header <regex> synopsis 28.4 [re.syn], just below the function template swap add two further overloads:
template <class charT, class traits> void swap(basic_regex<charT, traits>& e1, basic_regex<charT, traits>& e2); template <class charT, class traits> void swap(basic_regex<charT, traits>&& e1, basic_regex<charT, traits>& e2); template <class charT, class traits> void swap(basic_regex<charT, traits>& e1, basic_regex<charT, traits>&& e2);
In the class definition of basic_regex, just below 28.8 [re.regex]/3, perform the following changes:
Just after the copy c'tor:
basic_regex(basic_regex&&);
Just after the copy-assignment op.:
basic_regex& operator=(basic_regex&&);
Just after the first assign overload insert:
basic_regex& assign(basic_regex&& that);
Change the current swap function to read:
void swap(basic_regex&&);
In 28.8.2 [re.regex.construct], just below the copy c'tor add a corresponding member definition of:
basic_regex(basic_regex&& e);Effects: Move-constructs a basic_regex instance from e.
Postconditions: flags() and mark_count() return e.flags() and e.mark_count(), respectively, that e had before construction, leaving e in a valid state with an unspecified value.
Throws: nothing.
Also in 28.8.2 [re.regex.construct], just below the copy assignment c'tor add a corresponding member definition of:
basic_regex& operator=(basic_regex&& e);Effects: Returns the result of assign(std::move(e)).
In 28.8.3 [re.regex.assign], just below the first assign overload add a corresponding member definition of:
basic_regex& assign(basic_regex&& rhs);Effects: Move-assigns a basic_regex instance from rhs and returns *this.
Postconditions: flags() and mark_count() return rhs.flags() and rhs.mark_count(), respectively, that rhs had before assignment, leaving rhs in a valid state with an unspecified value.
Throws: nothing.
In 28.8.6 [re.regex.swap], change the signature of swap to say:
void swap(basic_regex&& e);
In 28.8.7.1 [re.regex.nmswap], just below the single binary swap function, add the two missing overloads:
template <class charT, class traits> void swap(basic_regex<charT, traits>&& e1, basic_regex<charT, traits>& e2); template <class charT, class traits> void swap(basic_regex<charT, traits>& e1, basic_regex<charT, traits>&& e2);
Section: 28.11.4 [re.alg.replace] Status: Open Submitter: Stephan T. Lavavej Date: 2007-09-22
View other active issues in [re.alg.replace].
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Discussion:
Two overloads of regex_replace() are currently provided:
template <class OutputIterator, class BidirectionalIterator,
class traits, class charT>
OutputIterator
regex_replace(OutputIterator out,
BidirectionalIterator first, BidirectionalIterator last,
const basic_regex<charT, traits>& e,
const basic_string<charT>& fmt,
regex_constants::match_flag_type flags =
regex_constants::match_default);
template <class traits, class charT>
basic_string<charT>
regex_replace(const basic_string<charT>& s,
const basic_regex<charT, traits>& e,
const basic_string<charT>& fmt,
regex_constants::match_flag_type flags =
regex_constants::match_default);
The absence of const charT * overloads prevents ordinary-looking code from compiling, such as:
const string s("kitten");
const regex r("en");
cout << regex_replace(s, r, "y") << endl;
The compiler error message will be something like "could not deduce template argument for 'const std::basic_string<_Elem> &' from 'const char[1]'".
Users expect that anything taking a basic_string<charT> can also take a const charT *. In their own code, when they write a function taking std::string (or std::wstring), they can pass a const char * (or const wchar_t *), thanks to basic_string's implicit constructor. Because the regex algorithms are templated on charT, they can't rely on basic_string's implicit constructor (as the compiler error message indicates, template argument deduction fails first).
If a user figures out what the compiler error message means, workarounds are available - but they are all verbose. Explicit template arguments could be given to regex_replace(), allowing basic_string's implicit constructor to be invoked - but charT is the last template argument, not the first, so this would be extremely verbose. Therefore, constructing a basic_string from each C string is the simplest workaround.
[ Sophia Antipolis: ]
We note that Boost already has these overloads. However, the proposed wording is provided only for 28.11.4 [re.alg.replace]; wording is needed for the synopsis as well. We also note that this has impact on match_results::format, which may require further overloads.
Proposed resolution:
Provide additional overloads for regex_replace(): one additional overload of the iterator-based form (taking const charT* fmt), and three additional overloads of the convenience form (one taking const charT* str, another taking const charT* fmt, and the third taking both const charT* str and const charT* fmt). 28.11.4 [re.alg.replace]:
template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const basic_string<charT>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);...
template <class traits, class charT> basic_string<charT> regex_replace(const basic_string<charT>& s, const basic_regex<charT, traits>& e, const basic_string<charT>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const basic_string<charT>& s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const basic_string<charT>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);
Section: 28.11.4 [re.alg.replace] Status: New Submitter: Stephan T. Lavavej Date: 2007-09-22
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Discussion:
regex_match() and regex_search() take const basic_string<charT, ST, SA>&. regex_replace() takes const basic_string<charT>&. This prevents regex_replace() from accepting basic_strings with custom traits and allocators.
Proposed resolution:
Overloads of regex_replace() taking basic_string should be additionally templated on class ST, class SA and take const basic_string<charT, ST, SA>&. Consistency with regex_match() and regex_search() would place class ST, class SA as the first template arguments; compatibility with existing code using TR1 and giving explicit template arguments to regex_replace() would place class ST, class SA as the last template arguments.
Section: 26.4.8.5.3 [rand.dist.samp.genpdf] Status: Open Submitter: Stephan Tolksdorf Date: 2007-09-21
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Duplicate of: 795
Discussion:
26.4.8.5.3 [rand.dist.samp.genpdf] describes the interface for a distribution template that is meant to simulate random numbers from any general distribution given only the density and the support of the distribution. I'm not aware of any general purpose algorithm that would be capable of correctly and efficiently implementing the described functionality. From what I know, this is essentially an unsolved research problem. Existing algorithms either require more knowledge about the distribution and the problem domain or work only under very limited circumstances. Even the state of the art special purpose library UNU.RAN does not solve the problem in full generality, and in any case, testing and customer support for such a library feature would be a nightmare.
Possible resolution: For these reasons, I propose to delete section 26.4.8.5.3 [rand.dist.samp.genpdf].
[ Bellevue: ]
Disagreement persists.
Objection to this issue is that this function takes a general functor. The general approach would be to normalize this function, integrate it, and take the inverse of the integral, which is not possible in general. An example function is sin(1+n*x) -- for any spatial frequency that the implementor chooses, there is a value of n that renders that choice arbitrarily erroneous.
Correction: The formula above should instead read 1+sin(n*x).
Objector proposes the following possible compromise positions:
- rand.dist.samp.genpdf takes an number of points so that implementor need not guess.
- replace rand.disk.samp.genpdf with an extension to either or both of the discrete functions to take arguments that take a functor and number of points in place of the list of probabilities. Reference issues 793 and 794.
Proposed resolution:
See N2424 for the proposed resolution.
Section: 20.6.4.3 [meta.unary.prop] Status: Open Submitter: Alisdair Meredith Date: 2007-10-10
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Discussion:
We have 3 separate type traits to identify classes supporting no-throw operations, which are very useful when trying to provide exception safety guarantees. However, I'm not entirely clear on what the current wording requires of a conforming implementation. To quote from has_nothrow_default_constructor:
or T is a class type with a default constructor that is known not to throw any exceptions
What level of magic do we expect to deduce if this is known?
E.g.
struct test{
int x;
test() : x() {}
};
Should I expect a conforming compiler to assert( has_nothrow_constructor<test>::value )
Is this a QoI issue?
Should I expect to 'know' only if-and-only-if there is an inline definition available?
Should I never expect that to be true, and insist that the user supplies an empty throw spec if they want to assert the no-throw guarantee?
It would be helpful to maybe have a footnote explaining what is required, but right now I don't know what to suggest putting in the footnote.
(agreement since is that trivial ops and explicit no-throws are required. Open if QoI should be allowed to detect further)
[ Bellevue: ]
This looks like a QoI issue. In the case of trivial and nothrow it is known. Static analysis of the program is definitely into QoI. Move to OPEN. Need to talk to Core about this.
Proposed resolution:
Section: 20.6.5 [meta.rel] Status: Open Submitter: Alisdair Meredith Date: 2007-10-10
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Discussion:
With the pending arrival of explicit conversion functions though, I'm wondering if we want an additional trait, is_explictly_convertible?
[ Bellevue: ]
Alisdair is considering preparing a paper listing a number of missing type traits, and feels that it might be useful to handle them all together rather than piecemeal. This would affect issue 719 and 750. These two issues should move to OPEN pending AM paper on type traits.
Proposed resolution:
Section: 23.2.7 [vector.bool] Status: Open Submitter: Alisdair Meredith Date: 2007-10-10
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Discussion:
A number of vector<bool> members take const bool& as arguments. Is there any chance we could change them to pass-by-value or would I be wasting everyone's time if wrote up an issue?
[ post Bellevue: ]
As we understand it, the original requester (Martin Sebor) would like for implementations to be permitted to pass-by-value. Alisdair suggests that if this is to be resolved, it should be resolved more generally, e.g. in other containers as well.
We note that this would break ABI. However, we also suspect that this might be covered under the "as-if" rule in section 1.9.
Many in the group feel that for vector<bool>, this is a "don't care", and that at this point in the process it's not worth the bandwidth.
Issue 679 -- which was in ready status pre-Bellevue and is now in the working paper -- is related to this, though not a duplicate.
Moving to Open with a task for Alisdair to craft a informative note to be put whereever appropriate in the WP. This note would clarify places where pass-by-const-ref can be transformed to pass-by-value under the as-if rule.
[ San Francisco: ]
This is really a clause 17 issue, rather than something specific to vector<bool>.
Move to Open. Alisdair to provide a resolution. Alternately, Howard can close this as NAD and then open a new issue to handle the general issue (rather than the vector<bool> one).
Howard: Haven't yet opened new issue. Lacking wording for it.
Proposed resolution:
Section: 20.1.2 [allocator.requirements] Status: Ready Submitter: Hans Boehm Date: 2007-10-11
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Discussion:
Did LWG recently discuss 20.1.2 [allocator.requirements]-2, which states that "All the operations on the allocators are expected to be amortized constant time."?
As I think I pointed out earlier, this is currently fiction for allocate() if it has to obtain memory from the OS, and it's unclear to me how to interpret this for construct() and destroy() if they deal with large objects. Would it be controversial to officially let these take time linear in the size of the object, as they already do in real life?
Allocate() more blatantly takes time proportional to the size of the object if you mix in GC. But it's not really a new problem, and I think we'd be confusing things by leaving the bogus requirements there. The current requirement on allocate() is generally not important anyway, since it takes O(size) to construct objects in the resulting space. There are real performance issues here, but they're all concerned with the constants, not the asymptotic complexity.
Proposed resolution:
Change 20.1.2 [allocator.requirements]/2:
-2- Table 39 describes the requirements on types manipulated through allocators.
All the operations on the allocators are expected to be amortized constant time.Table 40 describes the requirements on allocator types.
Section: 20.1.1 [utility.arg.requirements] Status: Ready Submitter: Yechezkel Mett Date: 2007-10-14
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Discussion:
The draft standard n2369 uses the term move constructor in a few places, but doesn't seem to define it.
MoveConstructible requirements are defined in Table 33 in 20.1.1 [utility.arg.requirements] as follows:
MoveConstructible requirements expression post-condition T t = rv t is equivalent to the value of rv before the construction [Note: There is no requirement on the value of rv after the construction. -- end note]
(where rv is a non-const rvalue of type T).
So I assume the move constructor is the constructor that would be used in filling the above requirement.
For vector::reserve, vector::resize and the vector modifiers given in 23.2.6.4 [vector.modifiers] we have
Requires: If value_type has a move constructor, that constructor shall not throw any exceptions.
Firstly "If value_type has a move constructor" is superfluous; every type which can be put into a vector has a move constructor (a copy constructor is also a move constructor). Secondly it means that for any value_type which has a throwing copy constructor and no other move constructor these functions cannot be used -- which I think will come as a shock to people who have been using such types in vector until now!
I can see two ways to correct this. The simpler, which is presumably what was intended, is to say "If value_type has a move constructor and no copy constructor, the move constructor shall not throw any exceptions" or "If value_type has a move constructor which changes the value of its parameter,".
The other alternative is add to MoveConstructible the requirement that the expression does not throw. This would mean that not every type that satisfies the CopyConstructible requirements also satisfies the MoveConstructible requirements. It would mean changing requirements in various places in the draft to allow either MoveConstructible or CopyConstructible, but I think the result would be clearer and possibly more concise too.
Proposed resolution:
Add new defintions to 17.3 [definitions]:
move constructor
a constructor which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the construction.
move assignment operator
an assignment operator which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the assignment.
move assignment
use of the move assignment operator.
[ Howard adds post-Bellevue: ]
Unfortunately I believe the wording recommended by the LWG in Bellevue is incorrect. reserve et. al. will use a move constructor if one is available, else it will use a copy constructor. A type may have both. If the move constructor is used, it must not throw. If the copy constructor is used, it can throw. The sentence in the proposed wording is correct without the recommended insertion. The Bellevue LWG recommended moving this issue to Ready. I am unfortunately pulling it back to Open. But I'm drafting wording to atone for this egregious action. :-)
Section: 20.8.13.2 [util.smartptr.shared] Status: Ready Submitter: Joe Gottman Date: 2007-10-31
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Discussion:
Consider the following program:
int main() {
shared_ptr<int> p(nullptr);
return 0;
}
This program will fail to compile because shared_ptr uses the following template constructor to construct itself from pointers:
template <class Y> shared_ptr(Y *);
According to N2431, the conversion from nullptr_t to Y * is not deducible, so the above constructor will not be found. There are similar problems with the constructors that take a pointer and a deleter or a pointer, a deleter and an allocator, as well as the corresponding forms of reset(). Note that N2435 will solve this problem for constructing from just nullptr, but not for constructors that use deleters or allocators or for reset().
In the case of the functions that take deleters, there is the additional question of what argument should be passed to the deleter when it is eventually called. There are two reasonable possibilities: nullptr or static_cast<T *>(0), where T is the template argument of the shared_ptr. It is not immediately clear which of these is better. If D::operator() is a template function similar to shared_ptr's constructor, then d(static_cast<T*>(0)) will compile and d(nullptr) will not. On the other hand, if D::operator()() takes a parameter that is a pointer to some type other that T (for instance U* where U derives from T) then d(nullptr) will compile and d(static_cast<T *>(0)) may not.
[ Bellevue: ]
The general idea is right, we need to be able to pass a nullptr to a shared_ptr, but there are a few borderline editorial issues here. (For example, the single-argument nullptr_t constructor in the class synopsis isn't marked explicit, but it is marked explicit in the proposed wording for 20.6.6.2.1. There is a missing empty parenthesis in the form that takes a nullptr_t, a deleter, and an allocator.)
More seriously: this issue says that a shared_ptr constructed from a nullptr is empty. Since "empty" is undefined, it's hard to know whether that's right. This issue is pending on handling that term better.
Peter suggests definition of empty should be "does not own anything"
Is there an editorial issue that post-conditions should refer to get() = nullptr, rather than get() = 0?
No strong feeling towards accept or NAD, but prefer to make a decision than leave it open.
Seems there are no technical merits between NAD and Ready, comes down to "Do we intentially want to allow/disallow null pointers with these functions". Staw Poll - support null pointers 5 - No null pointers 0
Move to Ready, modulo editorial comments
[ post Bellevue Peter adds: ]
The following wording changes are less intrusive:
In 20.8.13.2.1 [util.smartptr.shared.const], add:
shared_ptr(nullptr_t);after:
shared_ptr();(Absence of explicit intentional.)
px.reset( nullptr ) seems a somewhat contrived way to write px.reset(), so I'm not convinced of its utility.
It's similarly not clear to me whether the deleter constructors need to be extended to take nullptr, but if they need to:
Add
template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);after
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);Note that this changes the semantics of the new constructors such that they consistently call d(p) instead of d((T*)0) when p is nullptr.
The ability to be able to pass 0/NULL to a function that takes a shared_ptr has repeatedly been requested by users, but the other additions that the proposed resolution makes are not supported by real world demand or motivating examples.
It might be useful to split the obvious and non-controversial nullptr_t constructor into a separate issue. Waiting for "empty" to be clarified is unnecessary; this is effectively an alias for the default constructor.
[ Sophia Antipolis: ]
We want to remove the reset functions from the proposed resolution.
The remaining proposed resolution text (addressing the constructors) are wanted.
Disposition: move to review. The review should check the wording in the then-current working draft.
Proposed resolution:
In 20.8.13.2 [util.smartptr.shared] p4, add to the definition/synopsis of shared_ptr:
template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);
after
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
In 20.8.13.2.1 [util.smartptr.shared.const] add:
template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);
after
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
(reusing the following paragraphs 20.8.13.2.1 [util.smartptr.shared.const]/9-13 that speak of p.)
In 20.8.13.2.1 [util.smartptr.shared.const]/10, change
Effects: Constructs a shared_ptr object that owns thepointerobject p and the deleter d. The second constructor shall use a copy of a to allocate memory for internal use.
Rationale:
[ San Francisco: ]
"pointer" is changed to "object" to handle the fact that nullptr_t isn't a pointer.
Section: 23.1 [container.requirements] Status: Open Submitter: Paolo Carlini Date: 2007-11-11
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Discussion:
In an emplace member function the function parameter pack may be bound to a priori unlimited number of objects: some or all of them can be elements of the container itself. Apparently, in order to conform to the blanket statement 23.1 [container.requirements]/11, the implementation must check all of them for that possibility. A possible solution can involve extending the exception in 23.1 [container.requirements]/12 also to the emplace member. As a side note, the push_back and push_front member functions are luckily not affected by this problem, can be efficiently implemented anyway
[ Related to 767 ]
[ Bellevue: ]
The proposed addition (13) is partially redundant with the existing paragraph 12. Why was the qualifier "rvalues" added to paragraph 12? Why does it not cover subelements and pointers?
Resolution: Alan Talbot to rework language, then set state to Review.
Proposed resolution:
Add after 23.1 [container.requirements]/12:
-12- Objects passed to member functions of a container as rvalue references shall not be elements of that container. No diagnostic required.
-13- Objects bound to the function parameter pack of the emplace member function shall not be elements or sub-objects of elements of the container. No diagnostic required.
Section: 24.1 [iterator.concepts] Status: Review Submitter: Martin Sebor Date: 2007-12-14
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Discussion:
Issue 278 defines the meaning of the term "invalid iterator" as one that may be singular.
Consider the following code:
std::deque<int> x, y;
std::deque<int>::iterator i = x.end(), j = y.end();
x.swap(y);
Given that swap() is required not to invalidate iterators
and using the definition above, what should be the expected result of
comparing i and j to x.end()
and y.end(), respectively, after the swap()?
I.e., is the expression below required to evaluate
to true?
i == y.end() && j == x.end()
(There are at least two implementations where the expression
returns false.)
More generally, is the definition introduced in issue 278 meant to make any guarantees about whether iterators actually point to the same elements or be associated with the same containers after a non-invalidating operation as they did before?
Here's a motivating example intended to demonstrate the importance of the question:
Container x, y ({ 1, 2}); // pseudocode to initialize y with { 1, 2 }
Container::iterator i = y.begin() + 1;
Container::iterator j = y.end();
std::swap(x, y);
std::find(i, j, 3);
swap() guarantees that i and j
continue to be valid. Unless the spec says that even though they are
valid they may no longer denote a valid range the code above must be
well-defined. Expert opinions on this differ as does the behavior of
popular implementations for some standard Containers.
[ San Francisco: ]
Pablo: add a note to the last bullet of paragraph 11 of 23.1.1 clarifying that the end() iterator doesn't refer to an element and that it can therefore be invalidated.
Proposed wording:
[Note: The end() iterator does not refer to any element and can therefore be invalidated. -- end note]Howard will add this proposed wording to the issue and then move it to Review.
Proposed resolution:
Add to 23.1.1 [container.requirements.general], p11:
Unless otherwise specified (see 23.1.4.1, 23.1.5.1, 23.2.2.3, and 23.2.6.4) all container types defined in this Clause meet the following additional requirements:
- ...
- no swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. [Note: The end() iterator does not refer to any element and can therefore be invalidated. -- end note]
Section: 23 [containers] Status: Open Submitter: Alisdair Meredith Date: 2008-01-14
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Discussion:
It appears most containers declare but do not define a member-swap function.
This is unfortunate, as all overload the swap algorithm to call the member-swap function! (required for swappable guarantees [Table 37] and Container Requirements [Table 87])
Note in particular that Table 87 gives semantics of a.swap(b) as swap(a,b), yet for all containers we define swap(a,b) to call a.swap(b) - a circular definition.
A quick survey of clause 23 shows that the following containers provide a definition for member-swap:
array queue stack vector
Whereas the following declare it, but do not define the semantics:
deque list map multimap multiset priority_queue set unordered_map unordered_multi_map unordered_multi_set unordered_set
Suggested resolution:
Provide a definition for each of the affected containers...
[ Bellevue: ]
Move to Open and ask Alisdair to provide wording.
Proposed resolution:
Wording provided in N2590.
Section: 25.3.4 [alg.merge] Status: New Submitter: Daniel Krügler Date: 2008-01-25
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Discussion:
Though issue 283 has fixed many open issues, it seems that some are still open:
Both 25.3.4 [lib.alg.merge] in 14882:2003 and 25.3.4 [alg.merge] in N2461 have no Requires element and the Effects element contains some requirements, which is probably editorial. Worse is that:
Proposed resolution:
In 25.3.4 [alg.merge] replace p.1+ 2:
Effects:
MergesCopies all the elements of the two sorted ranges [first1,last1) and [first2,last2) into the range[result,result + (last1 - first1) + (last2 - first2))[result, last) (where last is equal to result + (last1 - first1) + (last2 - first2)), such that resulting range will be sorted in non-decreasing order; that is, for every iterator i in [result,last) other than result, the condition *i < *(i - 1) or, respectively, comp(*i, *(i - 1)) will be false.Requires: The resulting range shall not overlap with either of the original ranges.
The list will be sorted in non-decreasing order according to the ordering defined by comp; that is, for every iterator i in [first,last) other than first, the condition *i < *(i - 1) or comp(*i, *(i - 1)) will be false.The results of the expressions *first1 and *first2 shall be writable to the output iterator.
[N.B.: I attempted to reuse the wording style of inplace_merge, therefore proposing to insert ", respectively," between both predicate tests. This is no strictly necessary as other parts of <algorithm> show, just a matter of consistency]
Section: TR1 5.1.4.5 [tr.rand.eng.disc], TR1 5.1.4.6 [tr.rand.eng.xor] Status: New Submitter: John Maddock Date: 2008-01-15
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Discussion:
Table 16 of TR1 requires that all Pseudo Random Number generators have a
seed(integer-type s)
member function that is equivalent to:
mygen = Generator(s)
But the generators xor_combine and discard_block have no such seed member, only the
template <class Gen> seed(Gen&);
member, which will not accept an integer literal as an argument: something that appears to violate the intent of Table 16.
So... is this a bug in TR1?
This is a real issue BTW, since the Boost implementation does adhere to the requirements of Table 16, while at least one commercial implementation does not and follows a strict adherence to sections 5.1.4.5 and 5.1.4.6 instead.
[ Jens adds: ]
Both engines do have the necessary constructor, therefore the omission of the seed() member functions appears to be an oversight.
Proposed resolution:
Section: 24.5.1 [istream.iterator] Status: Ready Submitter: Martin Sebor Date: 2008-02-06
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Discussion:
The description of how an istream_iterator object becomes an end-of-stream iterator is a) ambiguous and b) out of date WRT issue 468:
istream_iterator reads (using operator>>) successive elements from the input stream for which it was constructed. After it is constructed, and every time ++ is used, the iterator reads and stores a value of T. If the end of stream is reached (operator void*() on the stream returns false), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments istream_iterator() always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of operator* on an end of stream is not defined. For any other iterator value a const T& is returned. The result of operator-> on an end of stream is not defined. For any other iterator value a const T* is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that ++ operators are not equality preserving, that is, i == j does not guarantee at all that ++i == ++j. Every time ++ is used a new value is read.
istream::operator void*() returns null if istream::fail() is true, otherwise non-null. istream::fail() returns true if failbit or badbit is set in rdstate(). Reaching the end of stream doesn't necessarily imply that failbit or badbit is set (e.g., after extracting an int from stringstream("123") the stream object will have reached the end of stream but fail() is false and operator void*() will return a non-null value).
Also I would prefer to be explicit about calling fail() here (there is no operator void*() anymore.)
Proposed resolution:
Change 24.5.1 [istream.iterator]/1:
istream_iterator reads (using operator>>) successive elements from the input stream for which it was constructed. After it is constructed, and every time ++ is used, the iterator reads and stores a value of T. Ifthe end of stream is reachedthe iterator fails to read and store a value of T (operator void*()fail() on the stream returnsfalsetrue), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments istream_iterator() always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of operator* on an end of stream is not defined. For any other iterator value a const T& is returned. The result of operator-> on an end of stream is not defined. For any other iterator value a const T* is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that ++ operators are not equality preserving, that is, i == j does not guarantee at all that ++i == ++j. Every time ++ is used a new value is read.
Section: 26.4.8.5.1 [rand.dist.samp.discrete] Status: Open Submitter: P.J. Plauger Date: 2008-02-09
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Discussion:
discrete_distribution should have a constructor like:
template<class _Fn>
discrete_distribution(result_type _Count, double _Low, double _High,
_Fn& _Func);
(Makes it easier to fill a histogram with function values over a range.)
[ Bellevue: ]
How do you specify the function so that it does not return negative values? If you do it is a bad construction. This requirement is already there. Where in each bin does one evaluate the function? In the middle. Need to revisit tomorrow.
[ Sophia Antipolis: ]
Bill is not requesting this.
Marc Paterno: _Fn cannot return negative values at the points where the function is sampled. It is sampled in the middle of each bin. _Fn cannot return 0 everywhere it is sampled.
Jens: lambda expressions are rvalues
Add a library issue to provide an initializer_list<double> constructor for discrete_distribution.
Marc Paterno: dislikes reference for _Fn parameter. Make it pass-by-value (to use lambda), use std::ref to wrap giant-state function objects.
Daniel: See random_shuffle, pass-by-rvalue-reference.
Daniel to draft wording.
[ Pre San Francisco, Daniel provided wording: ]
The here proposed changes of the WP refer to the current state of N2691. During the Sophia Antipolis meeting two different proposals came up regarding the functor argument type, either by value or by rvalue-reference. For consistence with existing conventions (state-free algorithms and the general_pdf_distribution c'tor signature) the author decided to propose a function argument that is provided by value. If severe concerns exists that stateful functions would be of dominant relevance, it should be possible to replace the two occurrences of Func by Func&& in this proposal as part of an editorial process.
Proposed resolution:
Non-concept version of the proposed resolution
In 26.4.8.5.1 [rand.dist.samp.discrete]/1, class discrete_distribution, just before the member declaration
explicit discrete_distribution(const param_type& parm);
insert:
template<typename Func> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);
Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::
template<typename Func> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);Complexity: Exactly nf invocations of fw.
Requires:
- fw shall be callable with one argument of type double, and shall return values of a type convertible to double;
- If nf > 0, the relation xmin < xmax shall hold, and for all sample values xk, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- The following relations shall hold: nf ≥ 0, and 0 < S = w0+. . .+wn-1.
Effects:
- If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value w0 = 1.
Otherwise, sets n = nf, deltax = (xmax - xmin)/n and xcent = xmin + 0.5 * deltax.
For each k = 0, . . . ,n-1, calculates: xk = xcent + k * deltax wk = fw(xk)Constructs a discrete_distribution object with probabilities:
pk = wk/S for k = 0, . . . , n-1.
Concept version of the proposed resolution
In 26.4.8.5.1 [rand.dist.samp.discrete]/1, class discrete_distribution, just before the member declaration
explicit discrete_distribution(const param_type& parm);
insert:
template<Callable<auto, double> Func> requires Convertible<Func::result_type, double> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);
Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::
template<Callable<auto, double> Func> requires Convertible<Func::result_type, double> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);Complexity: Exactly nf invocations of fw.
Requires:
- If nf > 0, the relation xmin < xmax shall hold, and for all sample values xk, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- The following relations shall hold: nf ≥ 0, and 0 < S = w0+. . .+wn-1.
Effects:
- If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value w0 = 1.
Otherwise, sets n = nf, deltax = (xmax - xmin)/n and xcent = xmin + 0.5 * deltax.
For each k = 0, . . . ,n-1, calculates: xk = xcent + k * deltax wk = fw(xk)Constructs a discrete_distribution object with probabilities:
pk = wk/S for k = 0, . . . , n-1.
Section: 26.4.8.5.2 [rand.dist.samp.pconst] Status: Open Submitter: P.J. Plauger Date: 2008-02-09
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Discussion:
piecewise_constant_distribution should have a constructor like:
template<class _Fn>
piecewise_constant_distribution(size_t _Count,
_Ty _Low, _Ty _High, _Fn& _Func);
(Makes it easier to fill a histogram with function values over a range. The two (reference 793) make a sensible replacement for general_pdf_distribution.)
[ Sophia Antipolis: ]
Marc: uses variable width of bins and weight for each bin. This is not giving enough flexibility to control both variables.
Add a library issue to provide an constructor taking an initializer_list<double> and _Fn for piecewise_constant_distribution.
Daniel to draft wording.
[ Pre San Francisco, Daniel provided wording. ]
The here proposed changes of the WP refer to the current state of N2691. For reasons explained in 793, the author decided to propose a function argument that is provided by value. The issue proposes a c'tor signature, that does not take advantage of the full flexibility of piecewise_constant_distribution, because it restricts on a constant bin width, but the use-case seems to be popular enough to justify it's introduction.
Proposed resolution:
Non-concept version of the proposed resolution
In 26.4.8.5.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution, just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert:
template<typename Func> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);
Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:
template<typename Func> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);[p5_1] Complexity: Exactly nf invocations of fw.
[p5_2] Requires:
- fw shall be callable with one argument of type RealType, and shall return values of a type convertible to double;
- For all sample values xk defined below, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- The following relations shall hold: xmin < xmax, and 0 < S = w0+. . .+wn-1.
[p5_3] Effects:
If nf == 0,
- sets deltax = xmax - xmin, and
- lets the sequence w have length n = 1 and consist of the single value w0 = 1, and
- lets the sequence b have length n+1 with b0 = xmin and b1 = xmax
Otherwise,
- sets n = nf, deltax = (xmax - xmin)/n, xcent = xmin + 0.5 * deltax, and
lets the sequences w and b have length n and n+1, resp. and
for each k = 0, . . . ,n-1, calculates: dxk = k * deltax bk = xmin + dxk xk = xcent + dxk wk = fw(xk),and
- sets bn = xmax
Constructs a piecewise_constant_distribution object with the above computed sequence b as the interval boundaries and with the probability densities:
ρk = wk/(S * deltax) for k = 0, . . . , n-1.[p5_4] [Note: In this context, the subintervals [bk, bk+1) are commonly known as the bins of a histogram. -- end note]
Concept version of the proposed resolution
In 26.4.8.5.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution, just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);
Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);[p5_1] Complexity: Exactly nf invocations of fw.
[p5_2] Requires:
- For all sample values xk defined below, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- The following relations shall hold: xmin < xmax, and 0 < S = w0+. . .+wn-1.
[p5_3] Effects:
If nf == 0,
- sets deltax = xmax - xmin, and
- lets the sequence w have length n = 1 and consist of the single value w0 = 1, and
- lets the sequence b have length n+1 with b0 = xmin and b1 = xmax
Otherwise,
- sets n = nf, deltax = (xmax - xmin)/n, xcent = xmin + 0.5 * deltax, and
lets the sequences w and b have length n and n+1, resp. and
for each k = 0, . . . ,n-1, calculates: dxk = k * deltax bk = xmin + dxk xk = xcent + dxk wk = fw(xk),and
- sets bn = xmax
Constructs a piecewise_constant_distribution object with the above computed sequence b as the interval boundaries and with the probability densities:
ρk = wk/(S * deltax) for k = 0, . . . , n-1.[p5_4] [Note: In this context, the subintervals [bk, bk+1) are commonly known as the bins of a histogram. -- end note]
Section: 26.4.7.1 [rand.util.seedseq] Status: Open Submitter: Stephan Tolksdorf Date: 2008-02-18
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Discussion:
The for-loop in the algorithm specification has n iterations, where n is defined to be end - begin, i.e. the number of supplied w-bit quantities. Previous versions of this algorithm and the general logic behind it suggest that this is an oversight and that in the context of the for-loop n should be the number of full 32-bit quantities in b (rounded upwards). If w is 64, the current algorithm throws away half of all bits in b. If w is 16, the current algorithm sets half of all elements in v to 0.
There are two more minor issues:
[ Bellevue: ]
Move to OPEN Bill will try to propose a resolution by the next meeting.
[ post Bellevue: Bill provided wording. ]
This issue is made moot if 803 is accepted.
Proposed resolution:
Replace 26.4.7.1 [rand.util.seedseq] paragraph 6 with:
Effects: Constructs a seed_seq object by effectively concatenating the low-order u bits of each of the elements of the supplied sequence [begin, end) in ascending order of significance to make a (possibly very large) unsigned binary number b having a total of n bits, and then carrying out the following algorithm:
for( v.clear(); n > 0; n -= 32 ) v.push_back(b mod 232), b /= 232;
Section: 20.5 [tuple] Status: Open Submitter: Lawrence Crowl Date: 2008-02-18
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Discussion:
Classes with trivial special member functions are inherently more efficient than classes without such functions. This efficiency is particularly pronounced on modern ABIs that can pass small classes in registers. Examples include value classes such as complex numbers and floating-point intervals. Perhaps more important, though, are classes that are simple collections, like pair and tuple. When the parameter types of these classes are trivial, the pairs and tuples themselves can be trivial, leading to substantial performance wins.
The current working draft make specification of trivial functions (where possible) much easer through defaulted and deleted functions. As long as the semantics of defaulted and deleted functions match the intended semantics, specification of defaulted and deleted functions will yield more efficient programs.
There are at least two cases where specification of an explicitly defaulted function may be desirable.
First, the std::pair template has a non-trivial default constructor, which prevents static initialization of the pair even when the types are statically initializable. Changing the definition to
pair() = default;
would enable such initialization. Unfortunately, the change is not semantically neutral in that the current definition effectively forces value initialization whereas the change would not value initialize in some contexts.
** Does the committee confirm that forced value initialization was the intent? If not, does the committee wish to change the behavior of std::pair in C++0x?
Second, the same default constructor issue applies to std::tuple. Furthermore, the tuple copy constructor is current non-trivial, which effectively prevents passing it in registers. To enable passing tuples in registers, the copy constructor should be make explicitly defaulted. The new declarations are:
tuple() = default; tuple(const tuple&) = default;
This changes is not implementation neutral. In particular, it prevents implementations based on pointers to the parameter types. It does however, permit implementations using the parameter types as bases.
** How does the committee wish to trade implementation efficiency versus implementation flexibility?
[ Bellevue: ]
General agreement; the first half of the issue is NAD.
Before voting on the second half, it was agreed that a "Strongly Favor" vote meant support for trivial tuples (assuming usual requirements met), even at the expense of other desired qualities. A "Weakly Favor" vote meant support only if not at the expense of other desired qualities.
Concensus: Go forward, but not at expense of other desired qualities.
It was agreed to Alisdair should fold this work in with his other pair/tuple action items, above, and that issue 801 should be "open", but tabled until Alisdair's proposals are disposed of.
Proposed resolution:
Section: 26.4.7.1 [rand.util.seedseq] Status: Ready Submitter: Charles Karney Date: 2008-02-22
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Discussion:
seed_seq(InputIterator begin, InputIterator end); constructs a seed_seq object repacking the bits of supplied sequence [begin, end) into a 32-bit vector.
This repacking triggers several problems:
I propose simplifying this seed_seq constructor to be "32-bit only". Despite it's being simpler, there is NO loss of functionality (see below).
Here's how the description would read
26.4.7.1 [rand.util.seedseq] Class seed_seq
template<class InputIterator> seed_seq(InputIterator begin, InputIterator end);5 Requires: NO CHANGE
6 Effects: Constructs a seed_seq object by
for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 2^32);
Discussion:
The chief virtues here are simplicity, portability, and generality.
Arguments (and counter-arguments) against making this change (and retaining the n2461 behavior) are:
The user can pass an array of unsigned char and seed_seq will nicely repack it.
Response: So what? Consider the seed string "ABC". The n2461 proposal results in
v = { 0x3, 0x434241 };
while the simplified proposal yields
v = { 0x41, 0x42, 0x43 };
The results produced by seed_seq::generate with the two inputs are different but nevertheless equivalently "mixed up" and this remains true even if the seed string is long.
With long strings (e.g., with bit-length comparable to the number of bits in the state), v is longer (by a factor of 4) with the simplified proposal and seed_seq::generate will be slower.
Response: It's unlikely that the efficiency of seed_seq::generate will be a big issue. If it is, the user is free to repack the seed vector before constructing seed_seq.
A user can pass an array of 64-bit integers and all the bits will be used.
Response: Indeed. However, there are many instances in the n2461 where integers are silently coerced to a narrower width and this should just be a case of the user needing to read the documentation. The user can of course get equivalent behavior by repacking his seed into 32-bit pieces. Furthermore, the unportability of the n2461 proposal with
unsigned long s[] = {1, 2, 3, 4};
seed_seq q(s, s+4);
which typically results in v = {1, 2, 3, 4} on 32-bit machines and in v = {1, 0, 2, 0, 3, 0, 4, 0} on 64-bit machines is a major pitfall for unsuspecting users.
Note: this proposal renders moot issues 782 and 800.
[ Bellevue: ]
Walter needs to ask Fermilab for guidance. Defer till tomorrow. Bill likes the proposed resolution.
[ Sophia Antipolis: ]
Marc Paterno wants portable behavior between 32bit and 64bit machines; we've gone to significant trouble to support portability of engines and their values.
Jens: the new algorithm looks perfectly portable
Marc Paterno to review off-line.
Modify the proposed resolution to read "Constructs a seed_seq object by the following algorithm ..."
Disposition: move to review; unanimous consent.
Proposed resolution:
Change 26.4.7.1 [rand.util.seedseq]:
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end);-5- Requires: InputIterator shall satisfy the requirements of an input iterator (24.1.1) such that iterator_traits<InputIterator>::value_type shall denote an integral type.
-6- Constructs a seed_seq object by the following algorithm
rearranging some or all of the bits of the supplied sequence [begin,end) of w-bit quantities into 32-bit units, as if by the following:
First extract the rightmost u bits from each of the n = end - begin elements of the supplied sequence and concatenate all the extracted bits to initialize a single (possibly very large) unsigned binary number, b = ∑n-1i=0 (begin[i] mod 2u) · 2w·i (in which the bits of each begin[i] are treated as denoting an unsigned quantity). Then carry out the following algorithm:v.clear(); if ($w$ < 32) v.push_back($n$); for( ; $n$ > 0; --$n$) v.push_back(b mod 232), b /= 232;for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);
Section: 27.6.4 [ext.manip] Status: New Submitter: Daniel Krügler Date: 2008-03-01
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Discussion:
The recent draft (as well as the original proposal n2072) uses an operational semantic for get_money ([ext.manip]/3) and put_money ([ext.manip]/5), which uses
istreambuf_iterator<charT>
and
ostreambuf_iterator<charT>
resp, instead of the iterator instances, with explicitly provided traits argument (The operational semantic defined by f is also traits dependent). This is an obvious oversight because both *stream_buf c'tors expect a basic_streambuf<charT,traits> as argument.
The same problem occurs within the get_time and put_time semantic (p. 7 and p. 9) of n2071 incorporated in N2521, where additional to the problem we have an editorial issue in get_time (streambuf_iterator instead of istreambuf_iterator).
Proposed resolution:
In 27.6.4 [ext.manip]/3 within function f replace the first line
template <class charT, class traits, class moneyT>
void f(basic_ios<charT, traits>& str, moneyT& mon, bool intl) {
typedef istreambuf_iterator<charT, traits> Iter;
...
In 27.6.4 [ext.manip]/4 remove the first template charT parameter:
template <class charT,class moneyT> unspecified put_money(const moneyT& mon, bool intl = false);
In 27.6.4 [ext.manip]/5 within function f replace the first line
template <class charT, class traits, class moneyT>
void f(basic_ios<charT, traits>& str, const moneyT& mon, bool intl) {
typedef ostreambuf_iterator<charT, traits> Iter;
...
In 27.6.4 [ext.manip]/7 within function f replace the first line
template <class charT, class traits>
void f(basic_ios<charT, traits>& str, struct tm *tmb, const charT *fmt) {
typedef istreambuf_iterator<charT, traits> Iter;
...
In 27.6.4 [ext.manip]/8 add const:
template <class charT> unspecified put_time(const struct tm *tmb, const charT *fmt);
In 27.6.4 [ext.manip]/9 within function f replace the first line
template <class charT, class traits>
void f(basic_ios<charT, traits>& str, const struct tm *tmb, const charT *fmt) {
typedef ostreambuf_iterator<charT, traits> Iter;
...
Add to the <iomanip> synopsis in 27.6 [iostream.format]
template <class moneyT> unspecified get_money(moneyT& mon, bool intl = false); template <class moneyT> unspecified put_money(const moneyT& mon, bool intl = false); template <class charT> unspecified get_time(struct tm *tmb, const charT *fmt); template <class charT> unspecified put_time(const struct tm *tmb, const charT *fmt);
Section: 23.2.7 [vector.bool] Status: Open Submitter: Alisdair Meredith Date: 2008-03-17
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Discussion:
vector<bool>::swap(reference, reference) has no definition.
[ San Francisco: ]
Move to Open. Alisdair to provide a resolution.
Proposed resolution:
Section: 20.7.16.2.4 [func.wrap.func.inv] Status: Open Submitter: Alisdair Meredith Date: 2008-03-16
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Discussion:
std::function and reference_closure should use "perfect forwarding" as described in the rvalue core proposal.
[ Sophia Antipolis: ]
According to Doug Gregor, as far as std::function is concerned, perfect forwarding can not be obtained because of type erasure. Not everyone agreed with this diagnosis of forwarding.
Proposed resolution:
Section: 20.7.12.1.3 [func.bind.bind] Status: Open Submitter: Stephan T. Lavavej Date: 2008-02-08
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Discussion:
Library Issue 527 notes that bind(f, t1, ..., tN) should be nofail when f, t1, ..., tN have nofail copy ctors.
However, no guarantees are provided for the copy ctor of the functor returned by bind(). (It's guaranteed to have a copy ctor, which can throw implementation-defined exceptions: bind() returns a forwarding call wrapper, TR1 3.6.3/2. A forwarding call wrapper is a call wrapper, TR1 3.3/4. Every call wrapper shall be CopyConstructible, TR1 3.3/4. Everything without an exception-specification may throw implementation-defined exceptions unless otherwise specified, C++03 17.4.4.8/3.)
Should the nofail guarantee requested by Library Issue 527 be extended to cover both calling bind() and copying the returned functor?
[ Howard adds: ]
tuple construction should probably have a similar guarantee.
[ San Francisco: ]
Howard to provide wording.
Proposed resolution:
Section: 20.7.12.1.3 [func.bind.bind] Status: Open Submitter: Howard Hinnant Date: 2008-03-17
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Discussion:
The functor retureed by bind() should have a move constructor that requires only move construction of its contained functor and bound arguments. That way move-only functors can be passed to objects such as thread.
This issue is related to issue 816.
[ San Francisco: ]
Howard to provide wording.
Proposed resolution:
Section: 18.7.6 [except.nested] Status: Open Submitter: Alisdair Meredith Date: 2008-03-25
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Discussion:
Looking at the wording I submitted for rethrow_if_nested, I don't think I got it quite right.
The current wording says:
template <class E> void rethrow_if_nested(const E& e);Effects: Calls e.rethrow_nested() only if e is publicly derived from nested_exception.
This is trying to be a bit subtle, by requiring e (not E) to be publicly derived from nested_exception the idea is that a dynamic_cast would be required to be sure. Unfortunately, if e is dynamically but not statically derived from nested_exception, e.rethrow_nested() is ill-formed.
[ San Francisco: ]
Alisdair was volunteered to provide wording.
Proposed resolution:
Section: 20.8.12.3.3 [unique.ptr.runtime.modifiers] Status: Ready Submitter: Alisdair Meredith Date: 2008-03-30
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Discussion:
Reading resolution of LWG issue 673 I noticed the following:
void reset(T*pointer p =0pointer());-1- Requires: Does not accept pointer types which are convertible to
T*pointer (diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]
This could be cleaned up by mandating the overload as a public deleted function. In addition, we should probably overload reset on nullptr_t to be a stronger match than the deleted overload. Words...
Proposed resolution:
Add to class template definition in 20.8.12.3 [unique.ptr.runtime]
// modifiers pointer release(); void reset(pointer p = pointer()); void reset( nullptr_t ); template< typename U > void reset( U ) = delete; void swap(unique_ptr&& u);
Update 20.8.12.3.3 [unique.ptr.runtime.modifiers]
void reset(pointer p = pointer()); void reset(nullptr_t);
-1- Requires: Does not accept pointer types which are convertible to pointer (diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]Effects: If get() == nullptr there are no effects. Otherwise get_deleter()(get()).
...
[ Note this wording incorporates resolutions for 806 (New) and 673 (Ready). ]
Section: 20.1.1 [utility.arg.requirements] Status: Review Submitter: James Kanze Date: 2008-04-01
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Discussion:
I just noticed that the following program is legal in C++03, but is forbidden in the current draft:
#include <vector>
#include <iostream>
class Toto
{
public:
Toto() {}
explicit Toto( Toto const& ) {}
} ;
int
main()
{
std::vector< Toto > v( 10 ) ;
return 0 ;
}
Is this change intentional? (And if so, what is the justification? I wouldn't call such code good, but I don't see any reason to break it unless we get something else in return.)
[ San Francisco: ]
The subgroup that looked at this felt this was a good change, but it may already be handled by incoming concepts (we're not sure).
Proposed resolution:
In 20.1.1 [utility.arg.requirements] change Table 33: MoveConstructible requirements [moveconstructible]:
expression post-condition T t(rv) = rvt is equivalent to the value of rv before the construction ...
In 20.1.1 [utility.arg.requirements] change Table 34: CopyConstructible requirements [copyconstructible]:
expression post-condition T t(u) = uthe value of u is unchanged and is equivalent to t ...
Section: 19.4.2.2 [syserr.errcode.overview], 20.8.13.2.8 [util.smartptr.shared.io], 22.2.8 [facets.examples], 20.3.6.3 [bitset.operators], 26.3.6 [complex.ops], 27.5 [stream.buffers], 28.9 [re.submatch] Status: Open Submitter: Alisdair Meredith Date: 2008-04-10
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Discussion:
Should the following use rvalues references to stream in insert/extract operators?
[ Sophia Antipolis ]
Agree with the idea in the issue, Alisdair to provide wording.
Proposed resolution:
Section: 20.8.13.2.1 [util.smartptr.shared.const] Status: Open Submitter: Peter Dimov Date: 2008-04-11
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Discussion:
Would anyone object to making the default constructor of shared_ptr (and weak_ptr and enable_shared_from_this) constexpr? This would enable static initialization for shared_ptr variables, eliminating another unfair advantage of raw pointers.
[ San Francisco: ]
It's not clear to us that you can initialize a pointer with the literal 0 in a constant expression. We need to ask CWG to make sure this works. Bjarne has been appointed to do this.
Core got back to us and assured as that nullptr would do the job nicely here.
Proposed resolution:
Section: 30.3.1.1 [thread.mutex.class] Status: Review Submitter: Peter Dimov Date: 2008-04-18
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Discussion:
[Note: I'm assuming here that 3.6.2 [basic.start.init]/1 will be fixed.]
Currently std::mutex doesn't support static initialization. This is a regression with respect to pthread_mutex_t, which does. I believe that we should strive to eliminate such regressions in expressive power where possible, both to ease migration and to not provide incentives to (or force) people to forego the C++ primitives in favor of pthreads.
[ Sophia Antipolis: ]
We believe this is implementable on POSIX, because the initializer-list feature and the constexpr feature make this work. Double-check core language about static initialization for this case. Ask core for a core issue about order of destruction of statically-initialized objects wrt. dynamically-initialized objects (should come afterwards). Check non-POSIX systems for implementability.
If ubiquitous implementability cannot be assured, plan B is to introduce another constructor, make this constexpr, which is conditionally-supported. To avod ambiguities, this new constructor needs to have an additional parameter.
Proposed resolution:
Change 30.3.1.1 [thread.mutex.class]:
class mutex {
public:
constexpr mutex();
...
Section: 21.1 [char.traits] Status: Open Submitter: Dietmar Kühl Date: 2008-04-23
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Discussion:
Paragraph 4 of 21.1 [char.traits] mentions that this
section specifies two specializations (char_traits<char>
and (char_traits<wchar_t>). However, there are actually
four specializations provided, i.e. in addition to the two above also
char_traits<char16_t> and char_traits<char32_t>).
I guess this was just an oversight and there is nothing wrong with just
fixing this.
[ Alisdair adds: ]
char_traits< char16/32_t > should also be added to <ios_fwd> in 27.2 [iostream.forward], and all the specializations taking a char_traits parameter in that header.
[ Sophia Antipolis: ]
Idea of the issue is ok.
Alisdair to provide wording, once that wording arrives, move to review.
Proposed resolution:
Replace paragraph 4 of 21.1 [char.traits] by:
This subclause specifies a struct template,
char_traits<charT>, and four explicit specializations of it,char_traits<char>,char_traits<char16_t>,char_traits<char32_t>, andchar_traits<wchar_t>, all of which appear in the header <string> and satisfy the requirements below.
Section: 17.6.2.4 [compliance] Status: Open Submitter: Beman Dawes Date: 2008-05-14
View all issues with Open status.
Discussion:
Once the C++0x standard library is feature complete, the LWG needs to review 17.6.2.4 [compliance] Freestanding implementations header list to ensure it reflects LWG consensus.
[ San Francisco: ]
This is a placeholder defect to remind us to review the table once we've stopped adding headers to the library.
Three new headers that need to be added to the list:
<initializer_list> <concept> <iterator_concepts><iterator_concepts>, in particular, has lots of stuff that isn't needed, so maybe the stuff that is needed should be broken out into a separate header.
Robert: What about reference_closure? It's currently in <functional>.
Proposed resolution:
Section: 20.8.12.2 [unique.ptr.single] Status: Open Submitter: Daniel Krügler Date: 2008-05-14
View all issues with Open status.
Discussion:
Issue 673 (including recent updates by 821) proposes a useful extension point for unique_ptr by granting support for an optional deleter_type::pointer to act as pointer-like replacement for element_type* (In the following: pointer).
Unfortunately no requirements are specified for the type pointer which has impact on at least two key features of unique_ptr:
Unique_ptr specification makes great efforts to require that essentially *all* operations cannot throw and therefore adds proper wording to the affected operations of the deleter as well. If user-provided pointer-emulating types ("smart pointers") will be allowed, either *all* throw-nothing clauses have to be replaced by weaker "An exception is thrown only if pointer's {op} throws an exception"-clauses or it has to be said explicitly that all used operations of pointer are required *not* to throw. I understand the main focus of unique_ptr to be as near as possible to the advantages of native pointers which cannot fail and thus strongly favor the second choice. Also, the alternative position would make it much harder to write safe and simple template code for unique_ptr. Additionally, I assume that a general statement need to be given that all of the expressions of pointer used to define semantics are required to be well-formed and well-defined (also as back-end for 762).
[ Sophia Antipolis: ]
Howard: We maybe need a core concept PointerLike, but we don't need the arithmetic (see shared_ptr vs. vector<T>::iterator.
Howard will go through and enumerate the individual requirements wrt. pointer for each member function.
Proposed resolution:
Add the following sentence just at the end of the newly proposed 20.8.12.2 [unique.ptr.single]/p. 3:
unique_ptr<T, D>::pointer's operations shall be well-formed, shall have well defined behavior, and shall not throw exceptions.
Section: 27.4.4.2 [basic.ios.members] Status: New Submitter: Martin Sebor Date: 2008-05-17
View other active issues in [basic.ios.members].
View all other issues in [basic.ios.members].
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Discussion:
The fix for issue 581, now integrated into the working paper, overlooks a couple of minor problems.
First, being an unformatted function once again, flush()
is required to create a sentry object whose constructor must, among
other things, flush the tied stream. When two streams are tied
together, either directly or through another intermediate stream
object, flushing one will also cause a call to flush() on
the other tied stream(s) and vice versa, ad infinitum. The program
below demonstrates the problem.
Second, as Bo Persson notes in his
comp.lang.c++.moderated post,
for streams with the unitbuf flag set such
as std::stderr, the destructor of the sentry object will
again call flush(). This seems to create an infinite
recursion for std::cerr << std::flush;
#include <iostream>
int main ()
{
std::cout.tie (&std::cerr);
std::cerr.tie (&std::cout);
std::cout << "cout\n";
std::cerr << "cerr\n";
}
Proposed resolution:
I think an easy way to plug the first hole is to add a requires clause
to ostream::tie(ostream *tiestr) requiring the this
pointer not be equal to any pointer on the list starting
with tiestr->tie()
through tiestr()->tie()->tie() and so on. I am not
proposing that we require implementations to traverse this list,
although I think we could since the list is unlikely to be very long.
Add a Requires clause to 27.4.4.2 [basic.ios.members] withethe following text:
Requires: If(tiestr != 0)is true,tiestrmust not be reachable by traversing the linked list of tied stream objects starting fromtiestr->tie().
In addition, to prevent the infinite recursion that Bo writes about in his comp.lang.c++.moderated post, I propose to change 27.6.2.4 [ostream::sentry], p2 like so:
If((os.flags() & ios_base::unitbuf) && !uncaught_exception())is true, callsos.flush()os.rdbuf()->pubsync().
money_base::space and
money_base::none on money_get
Section: 22.2.6.1.2 [locale.money.get.virtuals] Status: Open Submitter: Martin Sebor Date: 2008-05-17
View other active issues in [locale.money.get.virtuals].
View all other issues in [locale.money.get.virtuals].
View all issues with Open status.
Duplicate of: 670
Discussion:
In paragraph 2, 22.2.6.1.2 [locale.money.get.virtuals] specifies the following:
Wherespaceornoneappears in the format pattern, except at the end, optional white space (as recognized byct.is) is consumed after any required space.
This requirement can be (and has been) interpreted two mutually exclusive ways by different readers. One possible interpretation is that:
- where
money_base::spaceappears in the format, at least one space is required, and- where
money_base::noneappears in the format, space is allowed but not required.
The other is that:
where eithermoney_base::spaceormoney_base::noneappears in the format, white space is optional.
[ San Francisco: ]
Martin will revise the proposed resolution.
Proposed resolution:
I propose to change the text to make it clear that the first interpretation is intended, that is, to make following change to 22.2.6.1.2 [locale.money.get.virtuals], p2:
Whenmoney_base::spaceormoney_base::noneappears as the last element in the format pattern,except at the end, optional white space (as recognized byno white space is consumed. Otherwise, wherect.is) is consumed after any required space.money_base::spaceappears in any of the initial elements of the format pattern, at least one white space character is required. Wheremoney_base::noneappears in any of the initial elements of the format pattern, white space is allowed but not required. In either case, any required followed by all optional white space (as recognized byct.is()) is consumed. If(str.flags() & str.showbase)isfalse, ...
basic_ios::copyfmt() overly loosely specified
Section: 27.4.4.2 [basic.ios.members] Status: New Submitter: Martin Sebor Date: 2008-05-17
View other active issues in [basic.ios.members].
View all other issues in [basic.ios.members].
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Discussion:
The basic_ios::copyfmt() member function is specified in 27.4.4.2 [basic.ios.members] to have the following effects:
Effects: If(this == &rhs)does nothing. Otherwise assigns to the member objects of*thisthe corresponding member objects ofrhs, except that
rdstate()andrdbuf()are left unchanged;exceptions()is altered last by callingexceptions(rhs.except)- the contents of arrays pointed at by
pwordandiwordare copied not the pointers themselves
Since the rest of the text doesn't specify what the member objects
of basic_ios are this seems a little too loose.
Proposed resolution:
I propose to tighten things up by adding a Postcondition clause to the function like so:
Postconditions:
copyfmt()postconditionsElement Value rdbuf()unchanged tie()rhs.tie()rdstate()unchanged exceptions()rhs.exceptions()flags()rhs.flags()width()rhs.width()precision()rhs.precision()fill()rhs.fill()getloc()rhs.getloc()
The format of the table follows Table 117 (as
of N2588): basic_ios::init()
effects.
The intent of the new table is not to impose any new requirements or change existing ones, just to be more explicit about what I believe is already there.
Section: 24.5.1 [istream.iterator] Status: Open Submitter: Martin Sebor Date: 2008-05-17
View other active issues in [istream.iterator].
View all other issues in [istream.iterator].
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Discussion:
From message c++std-lib-20003...
The description of istream_iterator in
24.5.1 [istream.iterator], p1 specifies that objects of the
class become the end-of-stream (EOS) iterators under the
following condition (see also issue 788 another problem
with this paragraph):
If the end of stream is reached (operator void*()on the stream returnsfalse), the iterator becomes equal to the end-of-stream iterator value.
One possible implementation approach that has been used in practice is
for the iterator to set its in_stream pointer to 0 when
it reaches the end of the stream, just like the default ctor does on
initialization. The problem with this approach is that
the Effects clause for operator++() says the
iterator unconditionally extracts the next value from the stream by
evaluating *in_stream >> value, without checking
for (in_stream == 0).
Conformance to the requirement outlined in the Effects clause
can easily be verified in programs by setting eofbit
or failbit in exceptions() of the associated
stream and attempting to iterate past the end of the stream: each
past-the-end access should trigger an exception. This suggests that
some other, more elaborate technique might be intended.
Another approach, one that allows operator++() to attempt
to extract the value even for EOS iterators (just as long
as in_stream is non-0) is for the iterator to maintain a
flag indicating whether it has reached the end of the stream. This
technique would satisfy the presumed requirement implied by
the Effects clause mentioned above, but it isn't supported by
the exposition-only members of the class (no such flag is shown). This
approach is also found in existing practice.
The inconsistency between existing implementations raises the question
of whether the intent of the specification is that a non-EOS iterator
that has reached the EOS become a non-EOS one again after the
stream's eofbit flag has been cleared? That is, are the
assertions in the program below expected to pass?
sstream strm ("1 ");
istream_iterator eos;
istream_iterator it (strm);
int i;
i = *it++
assert (it == eos);
strm.clear ();
strm << "2 3 ";
assert (it != eos);
i = *++it;
assert (3 == i);
Or is it intended that once an iterator becomes EOS it stays EOS until the end of its lifetime?
[ San Francisco: ]
We like the direction of the proposed resolution. We're not sure about the wording, and we need more time to reflect on it,
Move to Open. Detlef to rewrite the proposed resolution in such a way that no reference is made to exposition only members of istream_iterator.
Proposed resolution:
The discussion of this issue on the reflector suggests that the intent
of the standard is for an istreambuf_iterator that has
reached the EOS to remain in the EOS state until the end of its
lifetime. Implementations that permit EOS iterators to return to a
non-EOS state may only do so as an extension, and only as a result of
calling istream_iterator member functions on EOS
iterators whose behavior is in this case undefined.
To this end we propose to change 24.5.1 [istream.iterator], p1, as follows:
The result of operator-> on an end-of-stream is not defined. For any other iterator value aconst T*is returned. Invokingoperator++()on an end-of-stream iterator is undefined. It is impossible to store things into istream iterators...
Add pre/postconditions to the member function descriptions of istream_iterator like so:
istream_iterator();Effects: Constructs the end-of-stream iterator.
Postcondition:in_stream == 0.istream_iterator(istream_type &s);Effects: Initializesin_streamwith &s. value may be initialized during construction or the first time it is referenced.
Postcondition:in_stream == &s.istream_iterator(const istream_iterator &x);Effects: Constructs a copy ofx.
Postcondition:in_stream == x.in_stream.istream_iterator& operator++();Requires:in_stream != 0.
Effects:*in_stream >> value.istream_iterator& operator++(int);Requires:in_stream != 0.
Effects:istream_iterator tmp (*this); *in_stream >> value; return tmp;
Section: 23.3 [associative], 23.4 [unord] Status: Open Submitter: Alan Talbot Date: 2008-05-18
View all issues with Open status.
Discussion:
Splice is a very useful feature of list. This functionality is also very useful for any other node based container, and I frequently wish it were available for maps and sets. It seems like an omission that these containers lack this capability. Although the complexity for a splice is the same as for an insert, the actual time can be much less since the objects need not be reallocated and copied. When the element objects are heavy and the compare operations are fast (say a map<int, huge_thingy>) this can be a big win.
Suggested resolution:
Add the following signatures to map, set, multimap, multiset, and the unordered associative containers:
void splice(list<T,Allocator>&& x); void splice(list<T,Allocator>&& x, const_iterator i); void splice(list<T,Allocator>&& x, const_iterator first, const_iterator last);
Hint versions of these are also useful to the extent hint is useful. (I'm looking for guidance about whether hints are in fact useful.)
void splice(const_iterator position, list<T,Allocator>&& x); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator first, const_iterator last);
[ Sophia Antipolis: ]
Don't try to splice "list" into the other containers, it should be container-type.
forward_list already has splice_after.
Would "splice" make sense for an unordered_map?
Jens, Robert: "splice" is not the right term, it implies maintaining ordering in lists.
Howard: adopt?
Jens: absorb?
Alan: subsume?
Robert: recycle?
Howard: transfer? (but no direction)
Jens: transfer_from. No.
Alisdair: Can we give a nothrow guarantee? If your compare() and hash() doesn't throw, yes.
Daniel: For unordered_map, we can't guarantee nothrow.
[ San Francisco: ]
Martin: this would possibly outlaw an implementation technique that is currently in use; caching nodes in containers.
Alan: if you cache in the allocator, rather than the individual container, this proposal doesn't interfere with that.
Martin: I'm not opposed to this, but I'd like to see an implementation that demonstrates that it works.
Proposed resolution:
Section: 21.3.1 [string.require] Status: Open Submitter: Hervé Brönnimann Date: 2008-06-05
View all other issues in [string.require].
View all issues with Open status.
Discussion:
In March, on comp.lang.c++.moderated, I asked what were the string exception safety guarantees are, because I cannot see *any* in the working paper, and any implementation I know offers the strong exception safety guarantee (string unchanged if a member throws exception). The closest the current draft comes to offering any guarantees is 21.3 [basic.string], para 3:
The class template basic_string conforms to the requirements for a Sequence Container (23.1.1), for a Reversible Container (23.1), and for an Allocator-aware container (91). The iterators supported by basic_string are random access iterators (24.1.5).
However, the chapter 23 only says, on the topic of exceptions: 23.1 [container.requirements], para 10:
Unless otherwise specified (see 23.2.2.3 and 23.2.6.4) all container types defined in this clause meet the following additional requirements:
- if an exception is thrown by...
I take it as saying that this paragraph has *no* implication on std::basic_string, as basic_string isn't defined in Clause 23 and this paragraph does not define a *requirement* of Sequence nor Reversible Container, just of the models defined in Clause 23. In addition, LWG Issue 718 proposes to remove 23.1 [container.requirements], para 3.
Finally, the fact that no operation on Traits should throw exceptions has no bearing, except to suggest (since the only other throws should be allocation, out_of_range, or length_error) that the strong exception guarantee can be achieved.
The reaction in that group by Niels Dekker, Martin Sebor, and Bo Persson, was all that this would be worth an LWG issue.
A related issue is that erase() does not throw. This should be stated somewhere (and again, I don't think that the 23.1 [container.requirements], para 1 applies here).
[ San Francisco: ]
Implementors will study this to confirm that it is actually possible.
Proposed resolution:
Add a blanket statement in 21.3.1 [string.require]:
- if any member function or operator of basic_string<charT, traits, Allocator> throws, that function or operator has no effect.
- no erase() or pop_back() function throws.
As far as I can tell, this is achieved by any implementation. If I made a mistake and it is not possible to offer this guarantee, then either state all the functions for which this is possible (certainly at least operator+=, append, assign, and insert), or add paragraphs to Effects clauses wherever appropriate.
Section: 23.2.1 [array] Status: Open Submitter: Benjamin Kosnik Date: 2008-06-05
View other active issues in [array].
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Discussion:
This is an issue that came up on the libstdc++ list, where a discrepancy between "C" arrays and C++0x's std::array was pointed out.
In "C," this array usage is possible:
int ar[] = {1, 4, 6};
But for C++,
std::array<int> a = { 1, 4, 6 }; // error
Instead, the second parameter of the array template must be explicit, like so:
std::array<int, 3> a = { 1, 4, 6 };
Doug Gregor proposes the following solution, that assumes generalized initializer lists.
template<typename T, typename... Args>
inline array<T, sizeof...(Args)>
make_array(Args&&... args)
{ return { std::forward<Args>(args)... }; }
Then, the way to build an array from a list of unknown size is:
auto a = make_array<T>(1, 4, 6);
[ San Francisco: ]
Benjamin: Move to Ready?
Bjarne: I'm not convinced this is useful enough to add, so I'd like us to have time to reflect on it.
Alisdair: the constraints are wrong, they should be
template<ValueType T, ValueType... Args> requires Convertible<Args, T>... array<T, sizeof...(Args)> make_array(Args&&... args);Alidair: this would be useful if we had a constexpr version.
Bjarne: this is probably useful for arrays with a small number of elements, but it's not clearly useful otherwise.
Consensus is to move to Open.
Proposed resolution:
Add to the array synopsis in 23.2 [sequences]:
template<ValueType T, ValueType... Args> requires Convertible<Args, T>... array<T, sizeof...(Args)> make_array(Args&&... args);
Append after 23.2.1.7 [array.tuple] Tuple interface to class template array the following new section.
23.2.1.7 Convenience interface to class template array [array.tuple]
template<ValueType T, ValueType... Args> requires Convertible<Args, T>... array<T, sizeof...(Args)> make_array(Args&&... args);Returns: an array<T, sizeof...(Args)> initialized with {std::forward<T>(args)...}.
Section: 20.3.6 [template.bitset] Status: Review Submitter: Howard Hinnant Date: 2008-06-18
View all other issues in [template.bitset].
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Discussion:
Issue 396 adds defaulted arguments to the to_string member, but neglects to update the three newer to_string overloads.
[ post San Francisco: ]
Daniel found problems with the wording and provided fixes. Moved from Ready to Review.
Proposed resolution:
replace in 20.3.6 [template.bitset]/1 (class bitset)
template <class charT, class traits>
basic_string<charT, traits, allocator<charT> >
to_string(charT zero = charT('0'), charT one = charT('1')) const;
template <class charT>
basic_string<charT, char_traits<charT>, allocator<charT> >
to_string(charT zero = charT('0'), charT one = charT('1')) const;
basic_string<char, char_traits<char>, allocator<char> >
to_string(char zero = '0', char one = '1') const;
replace in 20.3.6.2 [bitset.members]/37
template <class charT, class traits> basic_string<charT, traits, allocator<charT> > to_string(charT zero = charT('0'), charT one = charT('1')) const;37 Returns: to_string<charT, traits, allocator<charT> >(zero, one).
replace in 20.3.6.2 [bitset.members]/38
template <class charT> basic_string<charT, char_traits<charT>, allocator<charT> > to_string(charT zero = charT('0'), charT one = charT('1')) const;38 Returns: to_string<charT, char_traits<charT>, allocator<charT> >(zero, one).
replace in 20.3.6.2 [bitset.members]/39
basic_string<char, char_traits<char>, allocator<char> > to_string(char zero = '0', char one = '1') const;39 Returns: to_string<char, char_traits<char>, allocator<char> >(zero, one).
Section: 20.8.12.1.1 [unique.ptr.dltr.dflt] Status: Review Submitter: Howard Hinnant Date: 2008-06-18
View all issues with Review status.
Discussion:
No relationship between U and T in the converting constructor for default_delete template.
Requirements: U* is convertible to T* and has_virtual_destructor<T>; the latter should also become a concept.
Rules out cross-casting.
The requirements for unique_ptr conversions should be the same as those on the deleter.
[ Howard adds 2008-11-26: ]
I believe we need to be careful to not outlaw the following use case, and I believe the current proposed wording (requires Convertible<U*, T*> && HasVirtualDestructor<T>) does so:
#include <memory> int main() { std::unique_ptr<int> p1(new int(1)); std::unique_ptr<const int> p2(move(p1)); int i = *p2; // *p2 = i; // should not compile }I've removed "&& HasVirtualDestructor<T>" from the requires clause in the proposed wording.
Proposed resolution:
Change 20.8.12.1.1 [unique.ptr.dltr.dflt]:
namespace std {
template <class T> struct default_delete {
default_delete();
template <class U>
requires Convertible<U*, T*>
default_delete(const default_delete<U>&);
void operator()(T*) const;
};
}
...
template <class U> requires Convertible<U*, T*> default_delete(const default_delete<U>& other);
Section: 30.4.1 [thread.condition.condvar] Status: Open Submitter: Beman Dawes Date: 2008-06-13
View other active issues in [thread.condition.condvar].
View all other issues in [thread.condition.condvar].
View all issues with Open status.
Discussion:
The meaning of the bool returned by condition_variable::timed_wait is so obscure that even the class' designer can't deduce it correctly. Several people have independently stumbled on this issue.
It might be simpler to change the return type to a scoped enum:
enum class timeout { not_reached, reached };
That's the same cost as returning a bool, but not subject to mistakes. Your example below would be:
if (cv.wait_until(lk, time_limit) == timeout::reached ) throw time_out();
[ Beman to supply exact wording. ]
[ San Francisco: ]
There is concern that the enumeration names are just as confusing, if not more so, as the bool. You might have awoken because of a signal or a spurious wakeup, for example.
Group feels that this is a defect that needs fixing.
Group prefers returning an enum over a void return.
Howard to provide wording.
Proposed resolution:
Section: 30.4 [thread.condition] Status: Open Submitter: Pete Becker Date: 2008-06-23
View all issues with Open status.
Discussion:
N2661 says that there is a class named monotonic_clock. It also says that this name may be a synonym for system_clock, and that it's conditionally supported. So the actual requirement is that it can be monotonic or not, and you can tell by looking at is_monotonic, or it might not exist at all (since it's conditionally supported). Okay, maybe too much flexibility, but so be it.
A problem comes up in the threading specification, where several variants of wait_for explicitly use monotonic_clock::now(). What is the meaning of an effects clause that says
wait_until(lock, chrono::monotonic_clock::now() + rel_time)
when monotonic_clock is not required to exist?
[ San Francisco: ]
Nick: maybe instead of saying that chrono::monotonic_clock is conditionally supported, we could say that it's always there, but not necessarily supported..
Beman: I'd prefer a typedef that identifies the best clock to use for wait_for locks.
Tom: combine the two concepts; create a duration clock type, but keep the is_monotonic test.
Howard: if we create a duration_clock type, is it a typedef or an entirely true type?
There was broad preference for a typedef.
Move to Open. Howard to provide wording to add a typedef for duration_clock and to replace all uses of monotonic_clock in function calls and signatures with duration_clock.
[ Howard notes post-San Francisco: ]
After further thought I do not believe that creating a duration_clock typedef is the best way to proceed. An implementation may not need to use a time_point to implement the wait_for functions.
For example, on POSIX systems sleep_for can be implemented in terms of nanosleep which takes only a duration in terms of nanoseconds. The current working paper does not describe sleep_for in terms of sleep_until. And paragraph 2 of 30.1.4 [thread.req.timing] has the words strongly encouraging implementations to use monotonic clocks for sleep_for:
2 The member functions whose names end in _for take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions.I believe the approach taken in describing the effects of sleep_for and try_lock_for is also appropriate for wait_for. I.e. these are not described in terms of their _until variants.
Proposed resolution:
Change 30.4.1 [thread.condition.condvar], p21-22:
template <class Rep, class Period> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);Precondition: lock is locked by the calling thread, and either
- no other thread is waiting on this condition_variable object or
- lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting threads (via wait, wait_for or wait_until).
21 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time)
- Atomically calls lock.unlock() and blocks on *this.
- When unblocked, calls lock.lock() (possibly blocking on the lock) and returns.
- The function will unblock when signaled by a call to notify_one(), a call to notify_all(), by the elapsed time rel_time passing (30.1.4 [thread.req.timing]), or spuriously.
- If the function exits via an exception, lock.unlock() shall be called prior to exiting the function scope.
Postcondition: lock is locked by the calling thread.
22 Returns: false if the call is returning because the time duration specified by rel_time has elapsed, otherwise true.
Throws: std::system_error when the effects or postcondition cannot be achieved.
Error conditions:
- operation_not_permitted -- if the thread does not own the lock.
- equivalent error condition from lock.lock() or lock.unlock().
Change 30.4.1 [thread.condition.condvar], p26-p29:
template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);Precondition: lock is locked by the calling thread, and either
- no other thread is waiting on this condition_variable object or
- lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting threads (via wait, wait_for or wait_until).
26 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))
- Executes a loop: Within the loop the function first evaluates pred() and exits the loop if the result of pred() is true.
- Atomically calls lock.unlock() and blocks on *this.
- When unblocked, calls lock.lock() (possibly blocking on the lock).
- The function will unblock when signaled by a call to notify_one(), a call to notify_all(), by the elapsed time rel_time passing (30.1.4 [thread.req.timing]), or spuriously.
- If the function exits via an exception, lock.unlock() shall be called prior to exiting the function scope.
- The loop terminates when pred() returns true or when the time duration specified by rel_time has elapsed.
27 [Note: There is no blocking if pred() is initially true, even if the timeout has already expired. -- end note]
Postcondition: lock is locked by the calling thread.
28 Returns: pred()
29 [Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. -- end note]
Throws: std::system_error when the effects or postcondition cannot be achieved.
Error conditions:
- operation_not_permitted -- if the thread does not own the lock.
- equivalent error condition from lock.lock() or lock.unlock().
Change 30.4.2 [thread.condition.condvarany], p18-19:
template <class Lock, class Rep, class Period> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);18 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time)
- Atomically calls lock.unlock() and blocks on *this.
- When unblocked, calls lock.lock() (possibly blocking on the lock) and returns.
- The function will unblock when signaled by a call to notify_one(), a call to notify_all(), by the elapsed time rel_time passing (30.1.4 [thread.req.timing]), or spuriously.
- If the function exits via an exception, lock.unlock() shall be called prior to exiting the function scope.
Postcondition: lock is locked by the calling thread.
19 Returns: false if the call is returning because the time duration specified by rel_time has elapsed, otherwise true.
Throws: std::system_error when the returned value, effects, or postcondition cannot be achieved.
Error conditions:
- equivalent error condition from lock.lock() or lock.unlock().
Change 30.4.2 [thread.condition.condvarany], p23-p26:
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);Precondition: lock is locked by the calling thread, and either
- no other thread is waiting on this condition_variable object or
- lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting threads (via wait, wait_for or wait_until).
23 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))
- Executes a loop: Within the loop the function first evaluates pred() and exits the loop if the result of pred() is true.
- Atomically calls lock.unlock() and blocks on *this.
- When unblocked, calls lock.lock() (possibly blocking on the lock).
- The function will unblock when signaled by a call to notify_one(), a call to notify_all(), by the elapsed time rel_time passing (30.1.4 [thread.req.timing]), or spuriously.
- If the function exits via an exception, lock.unlock() shall be called prior to exiting the function scope.
- The loop terminates when pred() returns true or when the time duration specified by rel_time has elapsed.
24 [Note: There is no blocking if pred() is initially true, even if the timeout has already expired. -- end note]
Postcondition: lock is locked by the calling thread.
25 Returns: pred()
26 [Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. -- end note]
Throws: std::system_error when the effects or postcondition cannot be achieved.
Error conditions:
- operation_not_permitted -- if the thread does not own the lock.
- equivalent error condition from lock.lock() or lock.unlock().
Section: 26 [numerics] Status: Open Submitter: Lawrence Crowl Date: 2008-06-23
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Discussion:
There are a number of functions that affect the floating point state. These function need to be thread-safe, but I'm unsure of the right approach in the standard, as we inherit them from C.
[ San Francisco: ]
Nick: I think we already say that these functions do not introduce data races; see 17.6.5.6/20
Pete: there's more to it than not introducing data races; are these states maintained per thread?
Howard: 21.5/14 says that strtok and strerror are not required to avoid data races, and 20.9/2 says the same about asctime, gmtime, ctime, and gmtime.
Nick: POSIX has a list of not-safe functions. All other functions are implicitly thread safe.
Lawrence is to form a group between meetings to attack this issue. Nick and Tom volunteered to work with Lawrence.
Move to Open.
Proposed resolution:
Section: 23.1 [container.requirements] Status: Open Submitter: Daniel Krügler Date: 2008-06-24
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Discussion:
Table 89, Container requirements, defines operator== in terms of the container member function size() and the algorithm std::equal:
== is an equivalence relation. a.size() == b.size() && equal(a.begin(), a.end(), b.begin()
The new container forward_list does not provide a size member function by design but does provide operator== and operator!= without specifying it's semantic.
Other parts of the (sequence) container requirements do also depend on size(), e.g. empty() or clear(), but this issue explicitly attempts to solve the missing EqualityComparable specification, because of the special design choices of forward_list.
I propose to apply one of the following resolutions, which are described as:
Both proposal choices are discussed, the preferred choice of the author is to apply (A).
[ San Francisco: ]
There's an Option C: change the requirements table to use distance().
LWG found Option C acceptable.
Martin will draft the wording for Option C.
[ post San Francisco: ]
Martin provided wording for Option C.
Proposed resolution:
Common part:
Just betwen 23.2.3.5 [forwardlist.ops] and 23.2.3.6 [forwardlist.spec] add a new section "forwardlist comparison operators" [forwardlist.compare] (and also add the new section number to 23.2.3 [forwardlist]/2 in front of "Comparison operators"):
forwardlist comparison operators [forwardlist.compare]
Option (A):
Add to the new section [forwardlist.compare] the following paragraphs:
template <class T, class Allocator> bool operator==(const forward_list<T,Allocator>& x, const forward_list<T,Allocator>& y);Requires: Type T is EqualityComparable ([equalitycomparable]).
Returns: true if
for every iterator i in the range [x.begin(), E), where E == x.begin() + M and M == min(distance(x.begin(), x.end()), distance(y.begin(), y.end())), the following condition holds:
*i == *(y.begin() + (i - x.begin())).- if i == E then i == x.end() && (y.begin() + (i - x.begin())) == y.end().
- Otherwise, returns false.
Throws: Nothing unless an exception is thrown by the equality comparison.
Complexity: At most M comparisons.
template <class T, class Allocator> bool operator!=(const forward_list<T,Allocator>& x, const forward_list<T,Allocator>& y);Returns: !(x == y).
Option (B):
Add to the new section [forwardlist.compare] the following paragraphs:
template <class T, class Allocator> bool operator==(const forward_list<T,Allocator>& x, const forward_list<T,Allocator>& y);Requires: Type T is EqualityComparable ([equalitycomparable]).
Returns: distance(x.begin(), x.end()) == distance(y.begin(), y.end()) && equal(x.begin(), x.end(), y.begin()).
template <class T, class Allocator> bool operator!=(const forward_list<T,Allocator>& x, const forward_list<T,Allocator>& y);Returns: !(x == y).
Option (C):
Change Table 91 - Container Requirements in 23.1.1 [container.requirements.general] (N2798) like so:
Change the text in the Operational Semantics column in the row for a == b as follows:
== is an equivalence relation. distance(a.begin(), a.end())a.size()== distance(b.begin(), b.end())b.size()&& equal(a.begin(), a.end(), b.begin())Change the text in the Operational Semantics column in the row for a.max_size() as follows:
distance(a.begin(), a.end())a.size()of the largest possible containerChange the text in the Operational Semantics column in the row for a.empty() as follows:
a.begin() == a.end()a.size() == 0In addition, for consistency, change the text in the Operational Semantics column in the row for a.size() as follows:
distance(a.begin(), a.end())a.end() - a.begin()
Section: 25.3.5.1 [includes] Status: New Submitter: Alisdair Meredith Date: 2008-07-02
View all issues with New status.
Discussion:
In 25.3.5.1 [includes] the complexity is "at most -1 comparisons" if passed two empty ranges. I don't know how to perform a negative number of comparisions!
This same issue also applies to:
Proposed resolution:
Section: 27.8.1 [fstreams] Status: New Submitter: Steve Clamage Date: 2008-07-08
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Discussion:
Suppose writing to an [o]fstream fails and you later close the stream. The overflow() function is called to flush the buffer (if it exists). Then the file is unconditionally closed, as if by calling flcose.
If either overflow or fclose fails, close() reports failure, and clearly the stream should be in a failed or bad state.
Suppose the buffer is empty or non-existent (so that overflow() does not fail), and fclose succeeds. The close() function reports success, but what is the state of the stream?
Proposed resolution:
Section: 25.2.6 [alg.fill], 25.2.7 [alg.generate] Status: New Submitter: Daniel Krügler Date: 2008-07-13
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Discussion:
In regard to library defect 488 I found some more algorithms which unnecessarily throw away information. These are typically algorithms, which sequentially write into an OutputIterator, but do not return the final value of this output iterator. These cases are:
template<class OutputIterator, class Size, class T> void fill_n(OutputIterator first, Size n, const T& value);
template<class OutputIterator, class Size, class Generator> void generate_n(OutputIterator first, Size n, Generator gen);
In both cases the minimum requirements on the iterator are OutputIterator, which means according to the requirements of 24.1.3 [output.iterators]/2 that only single-pass iterations are guaranteed. So, if users of fill_n and generate_n have *only* an OutputIterator available, they have no chance to continue pushing further values into it, which seems to be a severe limitation to me.
Proposed resolution:
Replace the current declaration of fill_n in 25 [algorithms]/2, header <algorithm> synopsis and in 25.2.6 [alg.fill] by
template<class OutputIterator, class Size, class T>voidOutputIterator fill_n(OutputIterator first, Size n, const T& value);
Just after the effects clause p.2 add a new returns clause saying:
Returns: first + n for fill_n.
Replace the current declaration of generate_n in 25 [algorithms]/2, header <algorithm> synopsis and in 25.2.7 [alg.generate] by
template<class OutputIterator, class Size, class Generator>voidOutputIterator generate_n(OutputIterator first, Size n, Generator gen);
Just after the effects clause p.1 add a new returns clause saying:
Returns: first + n for generate_n.
Section: 20.8.11 [specialized.algorithms], 20.8.13.2.6 [util.smartptr.shared.create] Status: Ready Submitter: Alberto Ganesh Barbati Date: 2008-07-14
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Discussion:
LWG issue 402 replaced "new" with "::new" in the placement new-expression in 20.8.6.1 [allocator.members]. I believe the rationale given in 402 applies also to the following other contexts:
in 20.8.11 [specialized.algorithms], all four algorithms unitialized_copy, unitialized_copy_n, unitialized_fill and unitialized_fill_n use the unqualified placement new-expression in some variation of the form:
new (static_cast<void*>(&*result)) typename iterator_traits<ForwardIterator>::value_type(*first);
in 20.8.13.2.6 [util.smartptr.shared.create] there is a reference to the unqualified placement new-expression:
new (pv) T(std::forward<Args>(args)...),
I suggest to add qualification in all those places. As far as I know, these are all the remaining places in the whole library that explicitly use a placement new-expression. Should other uses come out, they should be qualified as well.
As an aside, a qualified placement new-expression does not need additional requirements to be compiled in a constrained context. By adding qualification, the HasPlacementNew concept introduced recently in N2677 (Foundational Concepts) would no longer be needed by library and should therefore be removed.
[ San Francisco: ]
Detlef: If we move this to Ready, it's likely that we'll forget about the side comment about the HasPlacementNew concept.
[ post San Francisco: ]
Daniel: HasPlacementNew has been removed from N2774 (Foundational Concepts).
Proposed resolution:
Replace "new" with "::new" in:
Section: 26.5.2.1 [valarray.cons] Status: New Submitter: Alberto Ganesh Barbati Date: 2008-07-20
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Discussion:
From 26.5.2.1 [valarray.cons], paragraph 2:
explicit valarray(size_t);The array created by this constructor has a length equal to the value of the argument. The elements of the array are constructed using the default constructor for the instantiating type T.
The problem is that the most obvious Ts for valarray are float and double, they don't have a default constructor. I guess the intent is to value-initialize the elements, so I suggest replacing:
The elements of the array are constructed using the default constructor for the instantiating type T.
with
The elements of the array are value-initialized.
There is another reference to the default constructor of T in the non-normative note in paragraph 9. That reference should also be replaced. (The normative wording in paragraph 8 refers to T() and so it doesn't need changes).
Proposed resolution:
Change 26.5.2.1 [valarray.cons], paragraph 2:
explicit valarray(size_t);The array created by this constructor has a length equal to the value of the argument. The elements of the array areconstructed using the default constructor for the instantiating type Tvalue-initialized (8.5 [dcl.init]).
Change 26.5.2.7 [valarray.members], paragraph 9:
[Example: If the argument has the value -2, the first two elements of the result will beconstructed using the default constructorvalue-initialized (8.5 [dcl.init]); the third element of the result will be assigned the value of the first element of the argument; etc. -- end example]
Section: 23 [containers] Status: Open Submitter: Alberto Ganesh Barbati Date: 2008-07-22
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Discussion:
The term "default constructed" is often used in wording that predates the introduction of the concept of value-initialization. In a few such places the concept of value-initialization is more correct than the current wording (for example when the type involved can be a built-in) so a replacement is in order. Two of such places are already covered by issue 867. This issue deliberately addresses the hopefully non-controversial changes in the attempt of being approved more quickly. A few other occurrences (for example in std::tuple, std::reverse_iterator and std::move_iterator) are left to separate issues. For std::reverse_iterator, see also issue 408. This issue is related with issue 724.
[ San Francisco: ]
The list provided in the proposed resolution is not complete. James Dennett will review the library and provide a complete list and will double-check the vocabulary.
This issue relates to Issue 886 tuple construction
Proposed resolution:
Change 20.1.1 [utility.arg.requirements], paragraph 2:
In general, a default constructor is not required. Certain container class member function signatures specifythe default constructorT() as a default argument. T() shall be a well-defined expression (8.5 [dcl.init]) if one of those signatures is called using the default argument (8.3.6 [dcl.fct.default]).
In all the following paragraphs in clause 23 [containers], replace "default constructed" with "value-initialized (8.5 [dcl.init])":
Section: 23.1.5 [unord.req] Status: Review Submitter: Sohail Somani Date: 2008-07-22
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Discussion:
Is there any language in the current draft specifying the behaviour of the following snippet?
unordered_set<int> s; unordered_set<int>::local_iterator it = s.end(0); // Iterate past end - the unspecified part it++;
I don't think there is anything about s.end(n) being considered an iterator for the past-the-end value though (I think) it should be.
[ San Francisco: ]
We believe that this is not a substantive change, but the proposed change to the wording is clearer than what we have now.
Proposed resolution:
Change Table 97 "Unordered associative container requirements" in 23.1.5 [unord.req]:
Table 97: Unordered associative container requirements expression return type assertion/note pre/post-condition complexity b.begin(n) local_iterator
const_local_iterator for const b.Pre: n shall be in the range [0,b.bucket_count()). Note: [b.begin(n), b.end(n)) is a valid range containing all of the elements in the nth bucket.b.begin(n) returns an iterator referring to the first element in the bucket. If the bucket is empty, then b.begin(n) == b.end(n).Constant b.end(n) local_iterator
const_local_iterator for const b.Pre: n shall be in the range [0, b.bucket_count()). b.end(n) returns an iterator which is the past-the-end value for the bucket. Constant
Section: 3.9.1 [basic.fundamental] Status: Open Submitter: Travis Vitek Date: 2008-06-30
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Discussion:
Neither the term "signed integral type" nor the term "unsigned integral type" is defined in the core language section of the standard, therefore the library section should avoid its use. The terms signed integer type and unsigned integer type are indeed defined (in 3.9.1 [basic.fundamental]), thus the usages should be replaced accordingly.
Note that the key issue here is that "signed" + "integral type" !=
"signed integral type".
The types bool, char, char16_t,
char32_t and wchar_t are all listed as
integral types, but are neither of signed integer type or
unsigned integer type. According to 3.9 [basic.types] p7, a synonym for
integral type is integer type.
Given this, one may choose to assume that an integral type that
can represent values less than zero is a signed integral type.
Unfortunately this can cause ambiguities.
As an example, if T is unsigned char, the
expression make_signed<T>::type, is supposed to
name a signed integral type. There are potentially two types that
satisfy this requirement, namely signed char and
char (assuming CHAR_MIN < 0).
[ San Francisco: ]
Plum, Sebor to review.
Proposed resolution:
I propose to use the terms "signed integer type" and "unsigned integer type" in place of "signed integral type" and "unsigned integral type" to eliminate such ambiguities.
The proposed change makes it absolutely clear that the difference between two pointers cannot be char or wchar_t, but could be any of the signed integer types. 5.7 [expr.add] paragraph 6...
- When two pointers to elements of the same array object are subtracted, the result is the difference of the subscripts of the two array elements. The type of the result is an implementation-defined
signed integral typesigned integer type; this type shall be the same type that is defined asstd::ptrdiff_tin the<cstdint>header (18.1)...
The proposed change makes it clear that X::size_type and X::difference_type cannot be char or wchar_t, but could be one of the signed or unsigned integer types as appropriate. 20.1.2 [allocator.requirements] table 40...
Table 40: Allocator requirements
expression return type assertion/note/pre/post-condition X::size_type unsigned integral typeunsigned integer typea type that can represent the size of the largest object in the allocation model. X::difference_type signed integral typesigned integer typea type that can represent the difference between any two pointers in the allocation model.
The proposed change makes it clear that make_signed<T>::type must be one of the signed integer types as defined in 3.9.1. Ditto for make_unsigned<T>type and unsigned integer types. 20.6.6.3 [meta.trans.sign] table 48...
Table 48: Sign modifications
Template Comments template <class T> struct make_signed; If Tnames a (possibly cv-qualified)signed integral typesigned integer type (3.9.1) then the member typedeftypeshall name the typeT; otherwise, ifTnames a (possibly cv-qualified)unsigned integral typeunsigned integer type thentypeshall name the correspondingsigned integral typesigned integer type, with the same cv-qualifiers asT; otherwise,typeshall name thesigned integral typesigned integer type with the smallest rank (4.13) for whichsizeof(T) == sizeof(type), with the same cv-qualifiers asT. Requires:Tshall be a (possibly cv-qualified) integral type or enumeration but not abooltype.template <class T> struct make_unsigned; If Tnames a (possibly cv-qualified)unsigned integral typeunsigned integer type (3.9.1) then the member typedeftypeshall name the typeT; otherwise, ifTnames a (possibly cv-qualified)signed integral typesigned integer type thentypeshall name the correspondingunsigned integral typeunsigned integer type, with the same cv-qualifiers asT; otherwise,typeshall name theunsigned integral typeunsigned integer type with the smallest rank (4.13) for whichsizeof(T) == sizeof(type), with the same cv-qualifiers asT. Requires:Tshall be a (possibly cv-qualified) integral type or enumeration but not abooltype.
Note: I believe that the basefield values should probably be prefixed with ios_base:: as they are in 22.2.2.2.2 [facet.num.put.virtuals] The listed virtuals are all overloaded on signed and unsigned integer types, the new wording just maintains consistency. 22.2.2.1.2 [facet.num.get.virtuals] table 78...
Table 78: Integer Conversions
State stdio equivalent basefield == oct %o basefield == hex %X basefield == 0 %i signed integral typesigned integer type%d unsigned integral typeunsigned integer type%u
Rationale is same as above. 22.2.2.2.2 [facet.num.put.virtuals] table 80...
Table 80: Integer Conversions
State stdio equivalent basefield == ios_base::oct %o (basefield == ios_base::hex) && !uppercase %x (basefield == ios_base::hex) %X basefield == 0 %i for a signed integral typesigned integer type%d for a unsigned integral typeunsigned integer type%u
23.1 [container.requirements] table 80...
Table 89: Container requirements
expression return type operational semantics assertion/note/pre/post-condition complexity X::difference_type signed integral typesigned integer typeis identical to the difference type of X::iterator and X::const_iterator compile time X::size_type unsigned integral typeunsigned integer typesize_type can represent any non-negative value of difference_type compile time
24.1 [iterator.concepts] paragraph 1...
Iterators are a generalization of pointers that allow a C++ program to work with different data structures (containers) in a uniform manner. To be able to construct template algorithms that work correctly and efficiently on different types of data structures, the library formalizes not just the interfaces but also the semantics and complexity assumptions of iterators. All input iteratorsisupport the expression*i, resulting in a value of some class, enumeration, or built-in typeT, called the value type of the iterator. All output iterators support the expression*i = owhereois a value of some type that is in the set of types that are writable to the particular iterator type ofi. All iteratorsifor which the expression(*i).mis well-defined, support the expressioni->mwith the same semantics as(*i).m. For every iterator typeXfor which equality is defined, there is a correspondingsigned integral typesigned integer type called the difference type of the iterator.
I'm a little unsure of this change. Previously this paragraph would allow instantiations of linear_congruential_engine on char, wchar_t, bool, and other types. The new wording prohibits this. 26.4.3.1 [rand.eng.lcong] paragraph 2...
The template parameterUIntTypeshall denote anunsigned integral typeunsigned integer type large enough to store values as large asm - 1. If the template parametermis 0, the modulusmused throughout this section 26.4.3.1 isnumeric_limits<result_type>::max()plus 1. [Note: The result need not be representable as a value of typeresult_type. --end note] Otherwise, the following relations shall hold:a < mandc < m.
Same rationale as the previous change. X [rand.adapt.xor] paragraph 6...
BothEngine1::result_typeandEngine2::result_typeshall denote (possibly different)unsigned integral typesunsigned integer types. The member result_type shall denote either the type Engine1::result_type or the type Engine2::result_type, whichever provides the most storage according to clause 3.9.1.
26.4.7.1 [rand.util.seedseq] paragraph 7...
Requires:RandomAccessIteratorshall meet the requirements of a random access iterator (24.1.5) such thatiterator_traits<RandomAccessIterator>::value_typeshall denote anunsigned integral typeunsigned integer type capable of accomodating 32-bit quantities.
By making this change, integral types that happen to have a signed representation, but are not signed integer types, would no longer be required to use a two's complement representation. This may go against the original intent, and should be reviewed. 29.4 [atomics.types.operations] paragraph 24...
Remark: Forsigned integral typessigned integer types, arithmetic is defined using two's complement representation. There are no undefined results. For address types, the result may be an undefined address, but the operations otherwise have no undefined behavior.
Section: 26.4.8.5.1 [rand.dist.samp.discrete] Status: New Submitter: Daniel Krügler Date: 2008-08-22
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Discussion:
During the Sophia Antipolis meeting it was decided to separate from 793 a subrequest that adds initializer list support to discrete_distribution, specifically, the issue proposed to add a c'tor taking a initializer_list<double>.
Proposed resolution:
In 26.4.8.5.1 [rand.dist.samp.discrete]/1, class discrete_distribution, just before the member declaration
explicit discrete_distribution(const param_type& parm);
insert
discrete_distribution(initializer_list<double> wl);
Between p.4 and p.5 of the same section insert a new paragraph as part of the new member description:
discrete_distribution(initializer_list<double> wl);Effects: Same as discrete_distribution(wl.begin(), wl.end()).
Section: 26.4.8.5.2 [rand.dist.samp.pconst] Status: New Submitter: Daniel Krügler Date: 2008-08-22
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Discussion:
During the Sophia Antipolis meeting it was decided to separate from 794 a subrequest that adds initializer list support to piecewise_constant_distribution, specifically, the issue proposed to add a c'tor taking a initializer_list<double> and a Callable to evaluate weight values. For consistency with the remainder of this class and the remainder of the initializer_list-aware library the author decided to change the list argument type to the template parameter RealType instead. For the reasoning to use Func instead of Func&& as c'tor function argument see issue 793.
Proposed resolution:
Non-concept version of the proposed resolution
In 26.4.8.5.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution, just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert
template<typename Func> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);
Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:
template<typename Func> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);[p5_1] Complexity: Exactly nf = max(bl.size(), 1) - 1 invocations of fw.
[p5_2] Requires:
- fw shall be callable with one argument of type RealType, and shall return values of a type convertible to double;
- The relation 0 < S = w0+. . .+wn-1 shall hold. For all sampled values xk defined below, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- If nf > 0 let bk = *(bl.begin() + k), k = 0, . . . , bl.size()-1 and the following relations shall hold for k = 0, . . . , nf-1: bk < bk+1.
[p5_3] Effects:
If nf == 0,
- lets the sequence w have length n = 1 and consist of the single value w0 = 1, and
- lets the sequence b have length n+1 with b0 = 0 and b1 = 1.
Otherwise,
- sets n = nf, and [bl.begin(), bl.end()) shall form the sequence b of length n+1, and
lets the sequences w have length n and for each k = 0, . . . ,n-1, calculates:
xk = 0.5*(bk+1 + bk) wk = fw(xk)Constructs a piecewise_constant_distribution object with the above computed sequence b as the interval boundaries and with the probability densities:
ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.
Concept version of the proposed resolution
In 26.4.8.5.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution, just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);
Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);[p5_1] Complexity: Exactly nf = max(bl.size(), 1) - 1 invocations of fw.
[p5_2] Requires:
- The relation 0 < S = w0+. . .+wn-1 shall hold. For all sampled values xk defined below, fw(xk) shall return a weight value wk that is non-negative, non-NaN, and non-infinity;
- If nf > 0 let bk = *(bl.begin() + k), k = 0, . . . , bl.size()-1 and the following relations shall hold for k = 0, . . . , nf-1: bk < bk+1.
[p5_3] Effects:
If nf == 0,
- lets the sequence w have length n = 1 and consist of the single value w0 = 1, and
- lets the sequence b have length n+1 with b0 = 0 and b1 = 1.
Otherwise,
- sets n = nf, and [bl.begin(), bl.end()) shall form the sequence b of length n+1, and
lets the sequences w have length n and for each k = 0, . . . ,n-1, calculates:
xk = 0.5*(bk+1 + bk) wk = fw(xk)Constructs a piecewise_constant_distribution object with the above computed sequence b as the interval boundaries and with the probability densities:
ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.
Section: 21.3 [basic.string] Status: Open Submitter: Daniel Krügler Date: 2008-08-22
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Discussion:
During the Sophia Antipolis meeting it was decided to split-off some parts of the n2647 ("Concurrency modifications for basic_string") proposal into a separate issue, because these weren't actually concurrency-related. The here proposed changes refer to the recent update document n2668 and attempt to take advantage of the stricter structural requirements.
Indeed there exists some leeway for more guarantees that would be very useful for programmers, especially if interaction with transactionary or exception-unaware C API code is important. This would also allow compilers to take advantage of more performance optimizations, because more functions can have throw() specifications. This proposal uses the form of "Throws: Nothing" clauses to reach the same effect, because there already exists a different issue in progress to clean-up the current existing "schizophrenia" of the standard in this regard.
Due to earlier support for copy-on-write, we find the following unnecessary limitations for C++0x:
The proposed resolution is split into a main part (A) and a secondary part (B) (earlier called "Adjunct Adjunct Proposal"). (B) extends (A) by also making access to index position size() of the at() overloads a no-throw operation. This was separated, because this part is theoretically observable in specifically designed test programs.
[ San Francisco: ]
We oppose part 1 of the issue but hope to address size() in issue 877.
We do not support part B. 4 of the issue because of the breaking API change.
We support part A. 2 of the issue.
On support part A. 3 of the issue:
Pete's broader comment: now that we know that basic_string will be a block of contiguous memory, we should just rewrite its specification with that in mind. The expression of the specification will be simpler and probably more correct as a result.
Proposed resolution:
In 21.3.4 [string.capacity], just after p. 1 add a new paragraph:
Throws: Nothing.
In 21.3.5 [string.access] replace p. 1 by the following 4 paragraghs:
Requires: pos ≤ size().
Returns: If pos < size(), returns *(begin() + pos). Otherwise, returns a reference to a charT() that shall not be modified.
Throws: Nothing.
Complexity: Constant time.
In 21.3.7.1 [string.accessors] replace the now common returns clause of c_str() and data() by the following three paragraphs:
Returns: A pointer p such that p+i == &operator[](i) for each i in [0, size()].
Throws: Nothing.
Complexity: Constant time.
In 21.3.5 [string.access] replace p.2 and p.3 by:
Requires: pos ≤ size()
Throws: out_of_range if pos > size().
Section: 17 [library] Status: Open Submitter: Martin Sebor Date: 2008-08-23
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Discussion:
Recent changes to the working draft have introduced a gratuitous inconsistency with the C++ 2003 version of the specification with respect to exception guarantees provided by standard functions. While the C++ 2003 standard consistenly uses the empty exception specification, throw(), to declare functions that are guaranteed not to throw exceptions, the current working draft contains a number of "Throws: Nothing." clause to specify essentially the same requirement. The difference between the two approaches is that the former specifies the behavior of programs that violate the requirement (std::unexpected() is called) while the latter leaves the behavior undefined.
A survey of the working draft reveals that there are a total of 209 occurrences of throw() in the library portion of the spec, the majority in clause 18, a couple (literally) in 19, a handful in 20, a bunch in 22, four in 24, one in 27, and about a dozen in D.9.
There are also 203 occurrences of "Throws: Nothing." scattered throughout the spec.
While sometimes there are good reasons to use the "Throws: Nothing." approach rather than making use of throw(), these reasons do not apply in most of the cases where this new clause has been introduced and the empty exception specification would be a better approach.
First, functions declared with the empty exception specification permit compilers to generate better code for calls to such functions. In some cases, the compiler might even be able to eliminate whole chunks of user-written code when instantiating a generic template on a type whose operations invoked from the template specialization are known not to throw. The prototypical example are the std::uninitialized_copy() and std::uninitialized_fill() algorithms where the entire catch(...) block can be optimized away.
For example, given the following definition of the std::uninitialized_copy function template and a user-defined type SomeType:
template <class InputIterator, class ForwardIterator>
ForwardIterator
uninitialized_copy (InputIterator first, InputIterator last, ForwardIterator res)
{
typedef iterator_traits<ForwardIterator>::value_type ValueType;
ForwardIterator start = res;
try {
for (; first != last; ++first, ++res)
::new (&*res) ValueType (*first);
}
catch (...) {
for (; start != res; --start)
(&*start)->~ValueType ();
throw;
}
return res;
}
struct SomeType {
SomeType (const SomeType&) throw ();
}
compilers are able to emit the following efficient specialization of std::uninitialized_copy<const SomeType*, SomeType*> (note that the catch block has been optimized away):
template <> SomeType*
uninitialized_copy (const SomeType *first, const SomeType *last, SomeType *res)
{
for (; first != last; ++first, ++res)
::new (res) SomeType (*first);
return res;
}
Another general example is default constructors which, when decorated with throw(), allow the compiler to eliminate the implicit try and catch blocks that it otherwise must emit around each the invocation of the constructor in new-expressions.
For example, given the following definitions of class MayThrow and WontThrow and the two statements below:
struct MayThrow {
MayThrow ();
};
struct WontThrow {
WontThrow () throw ();
};
MayThrow *a = new MayThrow [N];
WontThrow *b = new WontThrow [N];
the compiler generates the following code for the first statement:
MayThrow *a;
{
MayThrow *first = operator new[] (N * sizeof (*a));
MayThrow *last = first + N;
MayThrow *next = first;
try {
for ( ; next != last; ++next)
new (next) MayThrow;
}
catch (...) {
for ( ; first != first; --next)
next->~MayThrow ();
operator delete[] (first);
throw;
}
a = first;
}
but it is can generate much more compact code for the second statement:
WontThrow *b = operator new[] (N * sizeof (*b)); WontThrow *last = b + N; for (WontThrow *next = b; next != last; ++next) new (next) WontThrow;
Second, in order for users to get the maximum benefit out of the new std::has_nothrow_xxx traits when using standard library types it will be important for implementations to decorate all non throwing copy constructors and assignment operators with throw(). Note that while an optimizer may be able to tell whether a function without an explicit exception specification can throw or not based on its definition, it can only do so when it can see the source code of the definition. When it can't it must assume that the function may throw. To prevent violating the One Definition Rule, the std::has_nothrow_xxx trait must return the most pessimistic guess across all translation units in the program, meaning that std::has_nothrow_xxx<T>::value must evaluate to false for any T whose xxx (where xxx is default or copy ctor, or assignment operator) is defined out-of-line.
Counterarguments:
During the discussion of this issue on c++std-lib@accu.org (starting with post c++std-lib-21950) the following arguments in favor of the "Throws: Nothing." style have been made.
The answer to point (1) above is that implementers can (and some have) declare functions with throw() to indicate to the compiler that calls to the function can safely be assumed not to throw in order to allow it to generate efficient code at the call site without also having to define the functions the same way and causing the compiler to generate suboptimal code for the function definition. That is, the function is declared with throw() in a header but it's defined without it in the source file. The throw() declaration is suppressed when compiling the definition to avoid compiler errors. This technique, while strictly speaking no permitted by the language, is safe and has been employed in practice. For example, the GNU C library takes this approach. Microsoft Visual C++ takes a similar approach by simply assuming that no function with C language linkage can throw an exception unless it's explicitly declared to do so using the language extension throw(...).
Our answer to point (2) above is that there is no existing practice where C++ Standard Library implementers have opted to make use of the proprietary mechanisms to declare functions that don't throw. The language provides a mechanism specifically designed for this purpose. Avoiding its use in the specification itself in favor of proprietary mechanisms defeats the purpose of the feature. In addition, making use of the empty exception specification inconsistently, in some areas of the standard, while conspicuously avoiding it and making use of the "Throws: Nothing." form in others is confusing to users.
The answer to point (3) is simply to exercise caution when declaring functions and especially function templates with the empty exception specification. Functions that required not to throw but that may call back into user code are poor candidates for the empty exception specification and should instead be specified using "Throws: Nothing." clause.
Proposed resolution:
We propose two possible solutions. Our recommendation is to adopt Option 1 below.
Option 1:
Except for functions or function templates that make calls back to user-defined functions that may not be declared throw() replace all occurrences of the "Throws: Nothing." clause with the empty exception specification. Functions that are required not to throw but that make calls back to user code should be specified to "Throw: Nothing."
Option 2:
For consistency, replace all occurrences of the empty exception specification with a "Throws: Nothing." clause.
Section: 23.2.3 [forwardlist] Status: Review Submitter: Martin Sebor Date: 2008-08-23
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Discussion:
forward_list member functions that take a forward_list::iterator (denoted position in the function signatures) argument have the following precondition:
Requires: position is dereferenceable or equal to before_begin().
I believe what's actually intended is this:
Requires: position is in the range [before_begin(), end()).
That is, when it's dereferenceable, position must point into *this, not just any forward_list object.
[ San Francisco: ]
Robert suggested alternate proposed wording which had large support.
Proposed resolution:
Change the Requires clauses [forwardlist] , p21, p24, p26, p29, and, 23.2.3.5 [forwardlist.ops], p39, p43, p47 as follows:
Requires: position is before_begin() or a dereferenceable iterator in the range [begin(), end))or equal to before_begin(). ...
Section: 29 [atomics] Status: New Submitter: Alexander Chemeris Date: 2008-08-24
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Discussion:
The atomic_address type and atomic<T*> specialization provide atomic updates to pointers. However, the current specification requires that the types pointer be to non-const objects. This restriction is unnecessary and unintended.
Proposed resolution:
Add const qualification to the pointer values of the atomic_address and atomic<T*> specializations. E.g.
typedef struct atomic_address {
void store(const void*, memory_order = memory_order_seq_cst) volatile;
void* exchange( const void*, memory_order = memory_order_seq_cst) volatile;
bool compare_exchange( const void*&, const void*,
memory_order, memory_order) volatile;
bool compare_exchange( const void*&, const void*,
memory_order = memory_order_seq_cst ) volatile;
void* operator=(const void*) volatile;
} atomic_address;
void atomic_store(volatile atomic_address*, const void*);
void atomic_store_explicit(volatile atomic_address*, const void*,
memory_order);
void* atomic_exchange(volatile atomic_address*, const void*);
void* atomic_exchange_explicit(volatile atomic_address*, const void*,
memory_order);
bool atomic_compare_exchange(volatile atomic_address*,
const void**, const void*);
bool atomic_compare_exchange_explicit(volatile atomic_address*,
const void**, const void*,
memory_order, memory_order);
Section: 29 [atomics] Status: New Submitter: Lawrence Crowl Date: 2008-08-24
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Discussion:
The atomic_exchange and atomic_exchange_explicit functions seem to be inconsistently missing parameters.
Proposed resolution:
Add the appropriate parameters. For example,
bool atomic_exchange(volatile atomic_bool*, bool); bool atomic_exchange_explicit(volatile atomic_bool*, bool, memory_order);
Section: 20.8.13.2.1 [util.smartptr.shared.const] Status: Open Submitter: Peter Dimov Date: 2008-08-30
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Discussion:
We've changed shared_ptr<Y> to not convert to shared_ptr<T> when Y* doesn't convert to T* by resolving issue 687. This only fixed the converting copy constructor though. N2351 later added move support, and the converting move constructor is not constrained.
[ San Francisco: ]
We might be able to move this to NAD, Editorial once shared_ptr is conceptualized, but we want to revisit this issue to make sure.
Proposed resolution:
We need to change the Requires clause of the move constructor:
shared_ptr(shared_ptr&& r); template<class Y> shared_ptr(shared_ptr<Y>&& r);Requires:For the second constructor Y* shall be convertible to T*.The second constructor shall not participate in overload resolution unless Y* is convertible to T*.
in order to actually make the example in 687 compile (it now resolves to the move constructor).
Section: 23 [containers] Status: Open Submitter: Alisdair Meredith Date: 2008-09-10
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Discussion:
Note in particular that Table 90 "Container Requirements" gives semantics of a.swap(b) as swap(a,b), yet for all containers we define swap(a,b) to call a.swap(b) - a circular definition.
[ San Francisco: ]
Robert to propose a resolution along the lines of "Postcondition: "a = b, b = a" This will be a little tricky for the hash containers, since they don't have operator==.
Proposed resolution:
Section: 20.8.13.2.4 [util.smartptr.shared.mod] Status: Open Submitter: Jonathan Wakely Date: 2008-09-15
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Discussion:
#include <memory>
#include <cassert>
struct A { };
struct B : A { };
int main()
{
std::shared_ptr<A> pa(new A);
std::shared_ptr<B> pb(new B);
std::swap<A>(pa, pb); // N.B. no argument deduction
assert( pa.get() == pb.get() );
return 0;
}
Is this behaviour correct (I believe it is) and if so, is it unavoidable, or not worth worrying about?
This calls the lvalue/rvalue swap overload for shared_ptr:
template<class T> void swap( shared_ptr<T> & a, shared_ptr<T> && b );
silently converting the second argument from shared_ptr<B> to shared_ptr<A> and binding the rvalue ref to the produced temporary.
This is not, in my opinion, a shared_ptr problem; it is a general issue with the rvalue swap overloads. Do we want to prevent this code from compiling? If so, how?
Perhaps we should limit rvalue args to swap to those types that would benefit from the "swap trick". Or, since we now have shrink_to_fit(), just eliminate the rvalue swap overloads altogether. The original motivation was:
N1690.vector<A> v = ...; ... swap(v, vector<A>(v));
Proposed resolution:
Section: 20.3.3 [pairs] Status: Open Submitter: Alisdair Meredith Date: 2008-09-15
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Discussion:
20.2.3 pairs
Missing assignemnt operator:
template<class U , class V>
requires CopyAssignable<T1, U> && CopyAssignable<T2, V>
pair& operator=(pair<U , V> const & p );
Well, that's interesting. This assignment operator isn't in the current working paper, either. Perhaps we deemed it acceptable to build a temporary of type pair from pair<U, V>, then move-assign from that temporary?
It sounds more like an issue waiting to be opened, unless you want to plug it now. As written we risk moving from lvalues.
[ San Francisco: ]
Would be NAD if better ctors fixed it.
Related to 811.
[ post San Francisco: ]
Possibly NAD Editorial, solved by N2770.
Proposed resolution:
Section: 20.5.2.2 [tuple.cnstr] Status: Open Submitter: Alisdair Meredith Date: 2008-09-15
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Discussion:
20.5.2.2 [tuple.cnstr]:
Effects: Default initializes each element.
Could be clarified to state each "non-trivial" element. Otherwise we have a conflict with Core deinfition of default initialization - trivial types do not get initialized (rather than initialization having no effect)
I'm going to punt on this one, because it's not an issue that's related to concepts. I suggest bringing it to Howard's attention on the reflector.
[ San Francisco: ]
Text in draft doesn't mean anything, changing to "non-trivial" makes it meaningful.
We prefer "value initializes". Present implementations use value-initialization. Users who don't want value initialization have alternatives.
Request resolution text from Alisdair.
This issue relates to Issue 868 default construction and value-initialization.
Proposed resolution:
Section: 30.4.1 [thread.condition.condvar] Status: Open Submitter: Lawrence Crowl Date: 2008-09-15
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Discussion:
The Posix/C++ working group has identified an inconsistency between Posix and the C++ working draft in that Posix requires the clock to be identified at creation, whereas C++ permits identifying the clock at the call to wait. The latter cannot be implemented with the former.
[ San Francisco: ]
Howard recommends NAD with the following explanation:
The intent of the current wording is for the condtion_variable::wait_until be able to handle user-defined clocks as well as clocks the system knows about. This can be done by providing overloads for the known clocks, and another overload for unknown clocks which synchs to a known clock before waiting. For example:
template <class Duration> bool condition_variable::wait_until(unique_lock<mutex>& lock, const chrono::time_point<chrono::system_clock, Duration>& abs_time) { using namespace chrono; nanoseconds d = __round_up<nanoseconds>(abs_time.time_since_epoch()); __do_timed_wait(lock.mutex()->native_handle(), time_point<system_clock, nanoseconds>(d)); return system_clock::now() < abs_time; } template <class Clock, class Duration> bool condition_variable::wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time) { using namespace chrono; typename Clock::time_point c_entry = Clock::now(); system_clock::time_point s_entry = system_clock::now(); nanoseconds dn = __round_up<nanoseconds>(abs_time.time_since_epoch() - c_entry.time_since_epoch()); __do_timed_wait(lock.mutex()->native_handle(), s_entry + dn); return Clock::now() < abs_time; }In the above example, system_clock is the only clock which the underlying condition variable knows how to deal with. One overload just passes that clock through. The second overload (approximately) converts the unknown clock into a system_clock time_point prior to passing it down to the native condition variable.
On Posix systems vendors are free to add implementation defined constructors which take a clock. That clock can be stored in the condition_variable, and converted to (or not as necessary) as shown above.
If an implementation defined constructor takes a clock (for example), then part of the semantics for that implementation defined ctor might include that a wait_until using a clock other than the one constructed with results in an error (exceptional condition) instead of a conversion to the stored clock. Such a design is up to the vendor as once an implementation defined ctor is used, the vendor is free to specifiy the behavior of waits and/or notifies however he pleases (when the cv is constructed in an implementation defined manner).
Proposed resolution:
Section: 30.2.2 [thread.thread.this] Status: Review Submitter: Lawrence Crowl Date: 2008-09-15
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Discussion:
I never thought I'd say this, but this_thread::yield seems to be too strong in specification. The issue is that some systems distinguish between yielding to another thread in the same process and yielding to another process. Given that the C++ standard only talks about a single program, one can infer that the specification allows yielding only to another thread within the same program. Posix has no facility for that behavior. Can you please file an issue to weaken the wording. Perhaps "Offers the operating system the opportunity to reschedule."
Proposed resolution:
Change 30.2.2 [thread.thread.this]/3:
void this_thread::yield();Effects: Offers theoperating systemimplementation the opportunity to reschedule.another thread.
Section: 30.2.1.1 [thread.thread.id] Status: Open Submitter: Lawrence Crowl Date: 2008-09-15
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Discussion:
The thread::id type supports the full set of comparison operators. This is substantially more than is required for the associative containers that justified them. Please place an issue against the threads library.
[ San Francisco: ]
Would depend on proposed extension to POSIX, or non-standard extension. What about hash? POSIX discussing op. POSIX not known to be considering support needed for hash, op.
Group expresses support for putting ids in both unordered and ordered containers.
[ post San Francisco: ]
Howard: It turns out the current working paper N2723 already has hash<thread::id> (20.7 [function.objects], 20.7.17 [unord.hash]). We simply overlooked it in the meeting. It is a good thing we voted in favor of it (again). :-)
Recommend NAD.
Proposed resolution:
Section: 19.4.1 [syserr.errcat] Status: Open Submitter: Beman Dawes Date: 2008-09-14
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Discussion:
The static const error_category objects generic_category and system_category in header <system_error> are currently declared:
const error_category& get_generic_category(); const error_category& get_system_category(); static const error_category& generic_category = get_generic_category(); static const error_category& system_category = get_system_category();
This formulation has several problems:
The original plan was to eliminate these problems by applying the C++0x constexpr feature. See LWG issue 832. However, that approach turned out to be unimplementable, since it would require a constexpr object of a class with virtual functions, and that is not allowed by the core language.
The proposed resolution was developed as an alternative. It mitigates the above problems by removing initialization from the visible interface, allowing implementations to take advantage of static initialization if supported by a particular compiler.
Implementation experience:
Prototype implementations of the current WP interface and proposed resolution interface were tested with recent Codegear, GCC, Intel, and Microsoft compilers on Windows. The code generated by the Microsoft compiler was studied at length; the WP and proposal versions generated very similar code, but the WP version typically required three instructions per use, while the proposed version required two instructions per use. Both versions did make use of static initialization; apparently the compiler applied an implicit constexpr where useful, even in cases where constexpr would not be permitted by the language!
Acknowledgements:
Martin Sebor, Chris Kohlhoff, and John Lakos provided useful ideas and comments on initialization issues.
[ San Francisco: ]
Martin: prefers not to create more file-scope static objects, and would like to see get_* functions instead.
Beman: that implies a performance cost. Static initialization is quicker and more convenient than dynamic initialization.
Proposed resolution:
Assumes LWG issue 805 has been applied to the WP, resulting in posix_category being renamed generic_category. If 805 has not been applied, that name in this proposal must be adjusted accordingly.
Change 19.4.1.1 [syserr.errcat.overview] Class error_category overview error_category synopsis as indicated:
const error_category& get_generic_category();const error_category& get_system_category();staticstorage-class-specifier const error_category& generic_category= get_generic_category();staticstorage-class-specifier const error_category&native_categorysystem_category= get_system_category();
Change 19.4.1.5 [syserr.errcat.objects] Error category objects as indicated:
The implementation shall initialize generic_category and system_category. Their storage-class-specifier is permitted to be static or extern. It is unspecified if initialization is static or dynamic (3.6.2 [basic.start.init]). If initialization is dynamic, it shall occur before completion of the dynamic initialization of the first translation unit dynamically initialized that includes header <system_error>.
storage-class-specifier const error_category&get_generic_category();
Returns: A reference to an object of a type derived from class error_category.
Remarks:The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category. The object's name virtual function shall return a pointer to the string "GENERIC".storage-class-specifier const error_category&get_system_category();
Returns: A reference to an object of a type derived from class error_category.Remarks: The object's equivalent virtual functions shall behave as specified for class error_category. The object's name virtual function shall return a pointer to the string "system". The object's default_error_condition virtual function shall behave as follows:
If the argument ev corresponds to a POSIX errno value posv, the function shall return error_condition(posv, generic_category). Otherwise, the function shall return error_condition(ev, system_category). What constitutes correspondence for any given operating system is unspecified. [Note: The number of potential system error codes is large and unbounded, and some may not correspond to any POSIX errno value. Thus implementations are given latitude in determining correspondence. -- end note]
Section: 30.2.1.2 [thread.thread.constr], 30.3.5.2 [thread.once.callonce] Status: New Submitter: Peter Dimov Date: 2008-09-15
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Discussion:
I notice that the vararg overloads of std::thread and std::call_once (N2723 30.2.1.2 [thread.thread.constr] and 30.3.5.2 [thread.once.callonce]) are no longer specified in terms of std::bind; instead, some of the std::bind wording has been inlined into the specification.
There are two problems with this.
First, the specification (and implementation) in terms of std::bind allows, for example:
std::thread th( f, 1, std::bind( g ) );
which executes f( 1, g() ) in a thread. This can be useful. The "inlined" formulation changes it to execute f( 1, bind(g) ) in a thread.
Second, assuming that we don't want the above, the specification has copied the wording
INVOKE(func, w1, w2, ..., wN) (20.6.2) shall be a valid expression for some values w1, w2, ..., wN
but this is not needed since we know that our argument list is args; it should simply be
INVOKE(func, args...) (20.6.2) shall be a valid expression
Proposed resolution:
Section: 30.3.1.1 [thread.mutex.class] Status: New Submitter: Peter Dimov Date: 2008-09-15
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Discussion:
30.3.1.1 [thread.mutex.class]/27 (in N2723) says that the behavior is undefined if:
I don't believe that this is right. Calling lock() or try_lock() on a locked mutex is well defined in the general case. try_lock() is required to fail and return false. lock() is required to either throw an exception (and is allowed to do so if it detects deadlock) or to block until the mutex is free. These general requirements apply regardless of the current owner of the mutex; they should apply even if it's owned by the current thread.
Making double lock() undefined behavior probably can be justified (even though I'd still disagree with the justification), but try_lock() on a locked mutex must fail.
Proposed resolution:
Section: 18.9 [support.runtime] Status: Ready Submitter: Lawrence Crowl, Alisdair Meredith Date: 2008-09-17
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Discussion:
The interaction between longjmp and exceptions seems unnecessarily restrictive and not in keeping with existing practice.
Proposed resolution:
Edit paragraph 4 of 18.9 [support.runtime] as follows:
The function signature longjmp(jmp_buf jbuf, int val) has more restricted behavior in this International Standard. A setjmp/longjmp call pair has undefined behavior if replacing the setjmp and longjmp by catch and throw woulddestroyinvoke any non-trivial destructors for any automatic objects.
Section: 21.3.5 [string.access] Status: Open Submitter: James Dennett Date: 2008-09-16
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Discussion:
Per discussion, we need an issue open to cover looking at "Requires" clauses which are not constraints on user code, such as that on std::basic_string::at.
Proposed resolution:
Section: 20.8.13.2 [util.smartptr.shared] Status: Open Submitter: Hans Boehm Date: 2008-09-16
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Discussion:
It is unclear whether shared_ptr is thread-safe in the sense that multiple threads may simultaneously copy a shared_ptr. However this is a critical piece of information for the client, and it has significant impact on usability for many applications. (Detlef Vollman thinks it is currently clear that it is not thread-safe. Hans Boehm thinks it currently requires thread safety, since the use_count is not an explicit field, and constructors and assignment take a const reference to an existing shared_ptr.)
Pro thread-safety:
Many multi-threaded usages are impossible. A thread-safe version can be used to destroy an object when the last thread drops it, something that is often required, and for which we have no other easy mechanism.
Against thread-safety:
The thread-safe version is well-known to be far more expensive, even if used by a single thread. Many applications, including all single-threaded ones, do not care.
[ San Francisco: ]
Beman: this is a complicated issue, and would like to move this to Open and await comment from Peter Dimov; we need very careful and complete rationale for any decision we make; let's go slow
Detlef: I think that shared_ptr should not be thread-safe.
Hans: When you create a thread with a lambda, it in some cases makes it very difficult for the lambda to reference anything in the heap. It's currently ambiguous as to whether you can use a shared_ptr to get at an object.
Leave in Open. Detlef will submit an alternative proposed resolution that makes shared_ptr explicitly unsafe.
A third option is to support both threadsafe and non-safe share_ptrs, and to let the programmer decide which behavior they want.
Beman: Peter, do you support the PR?
Peter:
Yes, I support the proposed resolution, and I certainly oppose any attempts to make shared_ptr thread-unsafe.
I'd mildly prefer if
[Note: This is true in spite of that fact that such functions often modify use_count() --end note]is changed to
[Note: This is true in spite of that fact that such functions often cause a change in use_count() --end note](or something along these lines) to emphasise that use_count() is not, conceptually, a variable, but a return value.
Proposed resolution:
Make it explicitly thread-safe, in this weak sense, as I believe was intended:
Insert in 20.8.13.2 [util.smartptr.shared], before p5:
For purposes of determining the presence of a data race, member functions do not modify const shared_ptr and const weak_ptr arguments, nor any objects they refer to. [Note: This is true in spite of that fact that such functions often cause a change in use_count() --end note]
On looking at the text, I'm not sure we need a similar disclaimer anywhere else, since nothing else has the problem with the modified use_count(). I think Howard arrived at a similar conclusion.
Section: 23.2.3.4 [forwardlist.modifiers] Status: New Submitter: Howard Hinnant Date: 2008-09-22
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Discussion:
This issue was split off from 892 at the request of the LWG.
[ San Francisco: ]
This issue is more complicated than it looks.
paragraph 47: replace each (first, last) with (first, last]
add a statement after paragraph 48 that complexity is O(1)
remove the complexity statement from the first overload of splice_after
We may have the same problems with other modifiers, like erase_after. Should it require that all iterators in the range (position, last] be dereferenceable?
There are actually 3 issues here:
What value should erase_after return? With list, code often looks like:
for (auto i = l.begin(); i != l.end();)
{
// inspect *i and decide if you want to erase it
// ...
if (I want to erase *i)
i = l.erase(i);
else
++i;
}
I.e. the iterator returned from erase is useful for setting up the logic for operating on the next element. For forward_list this might look something like:
auto i = fl.before_begin();
auto ip1 = i;
for (++ip1; ip1 != fl.end(); ++ip1)
{
// inspect *(i+1) and decide if you want to erase it
// ...
if (I want to erase *(i+1))
i = fl.erase_after(i);
else
++i;
ip1 = i;
}
In the above example code, it is convenient if erase_after returns the element prior to the erased element (range) instead of the element after the erase element (range).
Existing practice:
There is not a strong technical argument for either solution over the other.
With all other containers, operations always work on the range [first, last) and/or prior to the given position.
With forward_list, operations sometimes work on the range (first, last] and/or after the given position.
This is simply due to the fact that in order to operate on *first (with forward_list) one needs access to *(first-1). And that's not practical with forward_list. So the operating range needs to start with (first, not [first (as the current working paper says).
Additionally, if one is interested in splicing the range (first, last), then (with forward_list), one needs practical (constant time) access to *(last-1) so that one can set the next field in this node to the proper value. As this is not possible with forward_list, one must specify the last element of interest instead of one past the last element of interest. The syntax for doing this is to pass (first, last] instead of (first, last).
With erase_after we have a choice of either erasing the range (first, last] or (first, last). Choosing the latter enables:
x.erase_after(pos, x.end());
With the former, the above statement is inconvenient or expensive due to the lack of constant time access to x.end()-1. However we could introduce:
iterator erase_to_end(const_iterator position);
to compensate.
The advantage of the former ((first, last]) for erase_after is a consistency with splice_after which uses (first, last] as the specified range. But this either requires the addition of erase_to_end or giving up such functionality.
Proposed resolution:
Wording below assumes issue 878 is accepted, but this issue is independent of that issue.
Change 23.2.3.4 [forwardlist.modifiers]:
iterator erase_after(const_iterator position);Requires: The iterator following position is dereferenceable.
Effects: Erases the element pointed to by the iterator following position.
Returns:
An iterator pointing to the element following the one that was erased, or end() if no such element existsAn iterator equal to position.iterator erase_after(const_iterator position, const_iterator last);Requires: All iterators in the range
[(position,last) are dereferenceable.Effects: Erases the elements in the range
[(position,last).Returns: An iterator equal to position
last
Change 23.2.3.5 [forwardlist.ops]:
void splice_after(const_iterator position, forward_list<T,Allocator>&& x);Requires: position is before_begin() or a dereferenceable iterator in the range [begin(), end)). &x != this.
Effects: Inserts the contents of x after position, and x becomes empty. Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.
Throws: Nothing.
Complexity:
Ο(1)Ο(distance(x.begin(), x.end()))...
void splice_after(const_iterator position, forward_list<T,Allocator>&& x, const_iterator first, const_iterator last);Requires: position is before_begin() or a dereferenceable iterator in the range [begin(), end)). (first,last]
)is a valid range in x, and all iterators in the range (first,last])are dereferenceable. position is not an iterator in the range (first,last]).Effects: Inserts elements in the range (first,last]
)after position and removes the elements from x. Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.Complexity: Ο(1).
Section: 23.2.3.5 [forwardlist.ops] Status: New Submitter: Arch Robison Date: 2008-09-08
View other active issues in [forwardlist.ops].
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Discussion:
I ran across a small contradiction in working draft n2723.
23.2.3 [forwardlist]p2: A forward_list satisfies all of the requirements of a container (table 90), except that the size() member function is not provided.
23.2.3.5 [forwardlist.ops]p57: Complexity: At most size() + x.size() - 1 comparisons.
Presumably 23.2.3.5 [forwardlist.ops]p57 needs to be rephrased to not use size(), or note that it is used there only for sake of notational convenience.
Proposed resolution:
Section: 20.8.13.2.2 [util.smartptr.shared.dest] Status: New Submitter: Peter Dimov Date: 2008-09-18
View all other issues in [util.smartptr.shared.dest].
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Discussion:
James Dennett, message c++std-lib-22442:
The wording below addresses one case of this, but opening an issue to address the need to sanity check uses of the term "pointer" in 20.8.13.2 [util.smartptr.shared] would be a good thing.
There's one more reference, in ~shared_ptr; we can apply your suggested change to it, too. That is:
Change 20.8.13.2.2 [util.smartptr.shared.dest]/1 second bullet from:
Otherwise, if *this owns a pointer p and a deleter d, d(p) is called.
to:
Otherwise, if *this owns an object p and a deleter d, d(p) is called.
Proposed resolution:
Change 20.8.13.2.2 [util.smartptr.shared.dest]/1 second bullet:
- ...
- Otherwise, if *this owns
a pointeran object p and a deleter d, d(p) is called.
Section: 27.8.1.8 [ifstream.assign] Status: New Submitter: Niels Dekker Date: 2008-09-20
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Discussion:
It appears that we have an issue similar to issue 675 regarding the move-assignment of stream types. For example, when assigning to an std::ifstream, ifstream1, it seems preferable to close the file originally held by ifstream1:
ifstream1 = std::move(ifstream2);
The current Draft (N2723) specifies that the move-assignment of stream types like ifstream has the same effect as a swap:
Assign and swap 27.8.1.8 [ifstream.assign]
basic_ifstream& operator=(basic_ifstream&& rhs);Effects: swap(rhs).
Proposed resolution:
Section: 24.4.2.5 [insert.iterator] Status: New Submitter: Alisdair Meredith Date: 2008-09-24
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Discussion:
The requires clause on the const T & overloads in back_insert_iterator/front_insert_iterator/insert_iterator mean that the assignment operator will implicitly move from lvalues of a move-only type.
Suggested resolutions are:
Proposed resolution:
Section: 18.2.1 [limits] Status: New Submitter: Alisdair Meredith Date: 2008-09-24
View all other issues in [limits].
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Discussion:
numeric_limits has functions specifically designed to return NaNs, which break the model of Regular (via its axioms.) While floating point types will be acceptible in many algorithms taking Regular values, it is not appopriate for this specific API and we need a less refined constraint.
Proposed resolution:
Section: 24.4.2.1 [back.insert.iterator] Status: New Submitter: Dave Abrahams Date: 2008-09-19
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Discussion:
I just noticed this; don't know how far the problem(?) extends or whether it's new or existing: back_insert_iterator's operator* is not const, so you can't dereference a const one.
Proposed resolution:
Section: 20.7.4 [func.ret] Status: New Submitter: Jonathan Wakely Date: 2008-09-10
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Discussion:
The WP and TR1 have the same text regarding the argument types of a result_of expression:
The values ti are lvalues when the corresponding type Ti is a reference type, and rvalues otherwise.
I read this to mean that this compiles:
typedef int (*func)(int&); result_of<func(int&&)>::type i = 0;
even though this doesn't:
int f(int&); f( std::move(0) );
Should the text be updated to say "when Ti is an lvalue-reference type" or am I missing something?
I later came up with this self-contained example which won't compile, but I think it should:
struct X {
void operator()(int&);
int operator()(int&&);
} x;
std::result_of< X(int&&) >::type i = x(std::move(0));
Proposed resolution:
Change 20.7.4 [func.ret], p1:
... The values ti are lvalues when the corresponding type Ti is an lvalue-reference type, and rvalues otherwise.
Section: 30.3.1.1 [thread.mutex.class] Status: New Submitter: Herb Sutter Date: 2008-09-18
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Discussion:
A few questions on the current WP, N2723:
30.3.1 [thread.mutex.requirements]/24 says an expression mut.unlock() "Throws: Nothing." I'm assuming that, per 17.6.4.11 [res.on.required], errors that violate the precondition "The calling thread shall own the mutex" opens the door for throwing an exception anyway, such as to report unbalanced unlock operations and unlocking from a thread that does not have ownership. Right?
30.3.1.1 [thread.mutex.class]/3 (actually numbered paragraph "27" in the WP; this is just a typo I think) says
The behavior of a program is undefined if:
- it destroys a mutex object owned by any thread,
- a thread that owns a mutex object calls lock() or try_lock() on that object, or
- a thread terminates while owning a mutex object.
As already discussed, I think the second bullet should be removed, and such a lock() or try_lock() should fail with an exception or returning false, respectively.
A potential addition to the list would be
but without that the status quo text endorses the technique of the program logically transferring ownership of a mutex to another thread with correctness enforced by programming discipline. Was that intended?
Proposed resolution:
Section: 18.8 [support.initlist] Status: New Submitter: Daniel Krügler Date: 2008-09-26
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Discussion:
The currently proposed constraint on initializer_list's element type E is that is has to meet ObjectType. This is an underspecification, because both core language and library part of initializer_list make clear, that it references an implicitly allocated array:
8.5.4 [dcl.init.list]/4:
When an initializer list is implicitly converted to a std::initializer_list<E>, the object passed is constructed as if the implementation allocated an array of N elements of type E, where N is the number of elements in the initializer list.[..]
18.8 [support.initlist]/2.
An object of type initializer_list<E> provides access to an array of objects of type const E.[..]
Therefore, E needs to fulfill concept ValueType (thus excluding abstract class types). This stricter requirement should be added to prevent deep instantiation errors known from the bad old times, as shown in the following example:
// Header A: (Should concept-check even in stand-alone modus)
template <DefaultConstructible T>
requires MoveConstructible<T>
void generate_and_do_3(T a) {
std::initializer_list<T> list{T(), std::move(a), T()};
...
}
void do_more();
void do_more_or_less();
template <DefaultConstructible T>
requires MoveConstructible<T>
void more_generate_3() {
do_more();
generate_and_do_3(T());
}
template <DefaultConstructible T>
requires MoveConstructible<T>
void something_and_generate_3() {
do_more_or_less();
more_generate_3();
}
// Test.cpp
#include "A.h"
class Abstract {
public:
virtual ~Abstract();
virtual void foo() = 0; // abstract type
Abstract(Abstract&&){} // MoveConstructible
Abstract(){} // DefaultConstructible
};
int main() {
// The restricted template *accepts* the argument, but
// causes a deep instantiation error in the internal function
// generate_and_do_3:
something_and_generate_3<Abstract>();
}
The proposed stricter constraint does not minimize the aim to support more general containers for which ObjectType would be sufficient. If such an extended container (lets assume it's still a class template) provides a constructor that accepts an initializer_list only this constructor would need to be restricted on ValueType:
template<ObjectType T>
class ExtContainer {
public:
requires ValueType<T>
ExtContainer(std::initializer_list<T>);
...
};
Proposed resolution:
Section: 20.3.6.2 [bitset.members] Status: New Submitter: Daniel Krügler Date: 2008-09-26
View all other issues in [bitset.members].
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Discussion:
The current standard 14882::2003(E) as well as the current draft N2723 have in common a contradiction of the operational semantics of member function test 20.3.6.2 [bitset.members]/56-58 and the immutable member operator[] overload 20.3.6.2 [bitset.members]/64-66 (all references are defined in terms of N2723):
bool test(size_t pos) const;
Requires: pos is valid
Throws: out_of_range if pos does not correspond to a valid bit position.
Returns: true if the bit at position pos in *this has the value one.
constexpr bool operator[](size_t pos) const;
Requires: pos shall be valid.
Throws: nothing.
Returns: test(pos).
Three interpretations:
The problem became worse, because issue 720 recently voted into WP argued that member test logically must be a constexpr function, because it was used to define the semantics of another constexpr function (the operator[] overload).
Three alternatives are proposed, corresponding to the three bullets (A), (B), and (C), the author suggests to follow proposal (C).
Proposed resolution:
Remove the constexpr specifier in front of operator[] overload and undo that of member test (assuming 720 is accepted) in both the class declaration 20.3.6 [template.bitset]/1 and in the member description before 20.3.6.2 [bitset.members]/56 and before /64 to read:
constexprbool test(size_t pos) const; ..constexprbool operator[](size_t pos) const;
Change the throws clause of p. 65 to read:
Throws:nothingout_of_range if pos does not correspond to a valid bit position.
Replace the throws clause p. 57 to read:
Throws:out_of_range if pos does not correspond to a valid bit positionnothing.
Undo the addition of the constexpr specifier to the test member function in both class declaration 20.3.6 [template.bitset]/1 and in the member description before 20.3.6.2 [bitset.members]/56, assuming that 720 was applied.
constexprbool test(size_t pos) const;
Change the returns clause p. 66 to read:
Returns:test(pos)true if the bit at position pos in *this has the value one, otherwise false.
Section: 29.3 [atomics.types] Status: New Submitter: Anthony Williams Date: 2008-09-26
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Discussion:
The deleted copy-assignment operators for the atomic types are not marked as volatile in N2723, whereas the assignment operators from the associated non-atomic types are. e.g.
atomic_bool& operator=(atomic_bool const&) = delete; atomic_bool& operator=(bool) volatile;
This leads to ambiguity when assigning a non-atomic value to a non-volatile instance of an atomic type:
atomic_bool b; b=false;
Both assignment operators require a standard conversions: the copy-assignment operator can use the implicit atomic_bool(bool) conversion constructor to convert false to an instance of atomic_bool, or b can undergo a qualification conversion in order to use the assignment from a plain bool.
This is only a problem once issue 845 is applied.
Proposed resolution:
Add volatile qualification to the deleted copy-assignment operator of all the atomic types:
atomic_bool& operator=(atomic_bool const&) volatile = delete; atomic_itype& operator=(atomic_itype const&) volatile = delete;
etc.
This will mean that the deleted copy-assignment operator will require two conversions in the above example, and thus be a worse match than the assignment from plain bool.
Section: 28.12.2 [re.tokiter] Status: New Submitter: Daniel Krügler Date: 2008-09-26
View all other issues in [re.tokiter].
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Discussion:
Construction of a regex_token_iterator (28.12.2 [re.tokiter]/6+) usually requires the provision of a sequence of integer values, which can currently be done via a std::vector<int> or a C array of int. Since the introduction of initializer_list in the standard it seems much more reasonable to provide a corresponding constructor that accepts an initializer_list<int> instead. This could be done as a pure addition or one could even consider replacement. The author suggests the replacement strategy (A), but provides an alternative additive proposal (B) as a fall-back, because of the handiness of this range type:
Proposed resolution:
In 28.12.2 [re.tokiter]/6 and the list 28.12.2.1 [re.tokiter.cnstr]/10-11 change the constructor declaration:
template <std::size_t N>regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re,const int (&submatches)[N]initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default);
In 28.12.2.1 [re.tokiter.cnstr]/12 change the last sentence
The third constructor initializes the member subs to hold a copy of the sequence of integer values pointed to by the iterator range [&submatches.begin(),&submatches.end()+ N).
In 28.12.2 [re.tokiter]/6 and the list 28.12.2.1 [re.tokiter.cnstr]/10-11 insert the following constructor declaration between the already existing ones accepting a std::vector and a C array of int, resp.:
regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
const regex_type& re,
initializer_list<int> submatches,
regex_constants::match_flag_type m =
regex_constants::match_default);
In 28.12.2.1 [re.tokiter.cnstr]/12 change the last sentence
The third and fourth constructor initializesthe member subs to hold a copy of the sequence of integer values pointed to by the iterator range [&submatches,&submatches + N) and [submatches.begin(),submatches.end()), respectively.
Section: 20.2.8 [concept.copymove] Status: New Submitter: Alberto Ganesh Barbati Date: 2008-09-29
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Discussion:
The description of the effect of operator= in the MoveAssignable concept, given in paragraph 7 is:
result_type T::operator=(T&& rv); // inherited from HasAssign<T, T&&>Postconditions: the constructed T object is equivalent to the value of rv before the assignment. [Note: there is no requirement on the value of rv after the assignment. --end note]
The sentence contains a typo (what is the "constructed T object"?) probably due to a cut&paste from MoveConstructible. Moreover, the discussion of LWG issue 675 shows that the postcondition is too generic and might not reflect the user expectations. An implementation of the move assignment that just calls swap() would always fulfill the postcondition as stated, but might have surprising side-effects in case the source rvalue refers to an object that is not going to be immediately destroyed. See LWG issue 900 for another example. Due to the sometimes intangible nature of the "user expectation", it seems difficult to have precise normative wording that could cover all cases without introducing unnecessary restrictions. However a non-normative clarification could be a very helpful warning sign that swapping is not always the correct thing to do.
Proposed resolution:
In 20.2.8 [concept.copymove], replace the postcondition in paragraph 7 with:
Postconditions: *this is equivalent to the value of rv before the assignment. [Note: there is no requirement on the value of rv after the assignment, but the effect should be unsurprising to the user even in case rv is not immediately destroyed. This may require that resources previously owned by *this are released instead of transferred to rv. -- end note]
Section: 27.6.1 [input.streams], 27.6.2 [output.streams] Status: New Submitter: Alberto Ganesh Barbati Date: 2008-09-29
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Discussion:
Class template basic_istream, basic_ostream and basic_iostream implements public move constructors, move assignment operators and swap method and free functions. This might induce both the user and the compiler to think that those types are MoveConstructible, MoveAssignable and Swappable. However, those class templates fail to fulfill the user expectations. For example:
std::ostream os(std::ofstream("file.txt"));
assert(os.rdbuf() == 0); // buffer object is not moved to os, file.txt has been closed
std::vector<std::ostream> v;
v.push_back(std::ofstream("file.txt"));
v.reserve(100); // causes reallocation
assert(v[0].rdbuf() == 0); // file.txt has been closed!
std::ostream&& os1 = std::ofstream("file1.txt");
os1 = std::ofstream("file2.txt");
os1 << "hello, world"; // still writes to file1.txt, not to file2.txt!
std::ostream&& os1 = std::ofstream("file1.txt");
std::ostream&& os2 = std::ofstream("file2.txt");
std::swap(os1, os2);
os1 << "hello, world"; // writes to file1.txt, not to file2.txt!
This is because the move constructor, the move assignment operator and swap are all implemented through calls to std::basic_ios member functions move() and swap() that do not move nor swap the controlled stream buffers. That can't happen because the stream buffers may have different types.
Notice that for basic_streambuf, the member function swap() is protected. I believe that is correct and all of basic_istream, basic_ostream, basic_iostream should do the same as the move ctor, move assignment operator and swap member function are needed by the derived fstreams and stringstreams template. The free swap functions for basic_(i|o|io)stream templates should be removed for the same reason.
Proposed resolution:
27.6.1.1 [istream]: make the following member functions protected:
basic_istream(basic_istream&& rhs); basic_istream& operator=(basic_istream&& rhs); void swap(basic_istream&& rhs);
Ditto: remove the three swap free functions signatures
// swap: template <class charT, class traits> void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y); template <class charT, class traits> void swap(basic_istream<charT, traits>&& x, basic_istream<charT, traits>& y); template <class charT, class traits> void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>&& y);
27.6.1.1.2 [istream.assign]: remove paragraph 4
template <class charT, class traits> void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y); template <class charT, class traits> void swap(basic_istream<charT, traits>&& x, basic_istream<charT, traits>& y); template <class charT, class traits> void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>&& y);Effects: x.swap(y).
27.6.1.5 [iostreamclass]: make the following member function protected:
basic_iostream(basic_iostream&& rhs); basic_iostream& operator=(basic_iostream&& rhs); void swap(basic_iostream&& rhs);
Ditto: remove the three swap free functions signatures
template <class charT, class traits> void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y); template <class charT, class traits> void swap(basic_iostream<charT, traits>&& x, basic_iostream<charT, traits>& y); template <class charT, class traits> void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>&& y);
27.6.1.5.3 [iostream.assign]: remove paragraph 3
template <class charT, class traits> void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y); template <class charT, class traits> void swap(basic_iostream<charT, traits>&& x, basic_iostream<charT, traits>& y); template <class charT, class traits> void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>&& y);Effects: x.swap(y).
27.6.2.1 [ostream]: make the following member function protected:
basic_ostream(basic_ostream&& rhs); basic_ostream& operator=(basic_ostream&& rhs); void swap(basic_ostream&& rhs);
Ditto: remove the three swap free functions signatures
// swap: template <class charT, class traits> void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y); template <class charT, class traits> void swap(basic_ostream<charT, traits>&& x, basic_ostream<charT, traits>& y); template <class charT, class traits> void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>&& y);
27.6.2.3 [ostream.assign]: remove paragraph 13 (The paragraphs seems to be misnumbered in the whole section 27.6.2 [output.streams] in N2723. The paragraph to remove is the one that describes the three swap free functions).
template <class charT, class traits> void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y); template <class charT, class traits> void swap(basic_ostream<charT, traits>&& x, basic_ostream<charT, traits>& y); template <class charT, class traits> void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>&& y);Effects: x.swap(y).
Section: 25.2.3 [alg.swap] Status: New Submitter: Daniel Krügler Date: 2008-10-01
View all other issues in [alg.swap].
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Discussion:
With the adaption of 809 we have a new algorithm swap for C-arrays, which needs to be conceptualized.
Proposed resolution:
Replace in 25.2.3 [alg.swap] before p. 3 until p. 4 by
template <classValueType T, size_t N> requires Swappable<T> void swap(T (&a)[N], T (&b)[N]);
Requires: T shall be Swappable.Effects: swap_ranges(a, a + N, b);
Section: 25.2.5 [alg.replace] Status: New Submitter: Daniel Krügler Date: 2008-10-03
View all other issues in [alg.replace].
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Discussion:
(A) 25.2.5 [alg.replace]/1:
Requires: The expression *first = new_value shall be valid.
(B) 25.2.5 [alg.replace]/4:
Requires: The results of the expressions *first and new_value shall be writable to the result output iterator.[..]
Since conceptualization, the quoted content of these clauses is covered by the existing requirements
(A) OutputIterator<Iter, const T&>
and
(B) OutputIterator<OutIter, InIter::reference> && OutputIterator<OutIter, const T&>
resp, and thus should be removed.
Proposed resolution:
Remove 25.2.5 [alg.replace]/1.
template<ForwardIterator Iter, class T> requires OutputIterator<Iter, Iter::reference> && OutputIterator<Iter, const T&> && HasEqualTo<Iter::value_type, T> void replace(Iter first, Iter last, const T& old_value, const T& new_value); template<ForwardIterator Iter, Predicate<auto, Iter::value_type> Pred, class T> requires OutputIterator<Iter, Iter::reference> && OutputIterator<Iter, const T&> && CopyConstructible<Pred> void replace_if(Iter first, Iter last, Pred pred, const T& new_value);1 Requires: The expression *first = new_value must be valid.
25.2.5 [alg.replace]/4: Remove the sentence "The results of the expressions *first and new_value shall be writable to the result output iterator.".
template<InputIterator InIter, typename OutIter, class T> requires OutputIterator<OutIter, InIter::reference> && OutputIterator<OutIter, const T&> && HasEqualTo<InIter::value_type, T> OutIter replace_copy(InIter first, InIter last, OutIter result, const T& old_value, const T& new_value); template<InputIterator InIter, typename OutIter, Predicate<auto, InIter::value_type> Pred, class T> requires OutputIterator<OutIter, InIter::reference> && OutputIterator<OutIter, const T&> && CopyConstructible<Pred> OutIter replace_copy_if(InIter first, InIter last, OutIter result, Pred pred, const T& new_value);4 Requires:The results of the expressions *first and new_value shall be writable to the result output iterator.The ranges [first,last) and [result,result + (last - first)) shall not overlap.
Section: 25.2.9 [alg.unique] Status: New Submitter: Daniel Krügler Date: 2008-10-03
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Discussion:
25.2.9 [alg.unique]/2: "Requires: The comparison function shall be an equivalence relation."
The essence of this is already covered by the given requirement
EquivalenceRelation<auto, Iter::value_type> Pred
and should thus be removed.
Proposed resolution:
Remove 25.2.9 [alg.unique]/2
template<ForwardIterator Iter> requires OutputIterator<Iter, Iter::reference> && EqualityComparable<Iter::value_type> Iter unique(Iter first, Iter last); template<ForwardIterator Iter, EquivalenceRelation<auto, Iter::value_type> Pred> requires OutputIterator<Iter, RvalueOf<Iter::reference>::type> && CopyConstructible<Pred> Iter unique(Iter first, Iter last, Pred pred);1 Effects: ...
2 Requires: The comparison function shall be an equivalence relation.
Section: 25.3.7 [alg.min.max] Status: New Submitter: Daniel Krügler Date: 2008-10-04
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Discussion:
It seems that the proposed changes for N2772 were not clear enough in this point:
25.3.7 [alg.min.max], before p.23 + p.24 + before p. 27 + p. 28 say that the return type of the minmax overloads with an initializer_list is pair<const T&, const T&>, which is inconsistent with the decision for the other min/max overloads which take a initializer_list as argument and return a T, not a const T&. Doing otherwise for minmax would easily lead to unexpected life-time problems by using minmax instead of min and max separately.
Proposed resolution:
In 25 [algorithms]/2, Header <algorithm> synopsis and in 25.3.7 [alg.min.max], before p.23 + p.24 + before p. 27 + p. 28 replace all occurrences ofpair<const T&, const T&>
by
pair<T, T>
as return types of minmax(initializer_list<T>) and minmax(initializer_list<T>, Compare comp), resp.
template<class T> pair<constT&,constT&> minmax(initializer_list<T> t);...
24 Returns: pair<
constT&,constT&>(x, y) where x ...template<class T, class Compare> pair<constT&,constT&> minmax(initializer_list<T> t, Compare comp);...
28 Returns: pair<constT&,constT&>(x, y) where x ...
Section: 20.3.3 [pairs] Status: New Submitter: Daniel Krügler Date: 2008-10-04
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Discussion:
The current WP provides the following assignment operators for pair in 20.3.3 [pairs]/1:
template<class U , class V> requires HasAssign<T1, const U&> && HasAssign<T2, const V&> pair& operator=(const pair<U , V>& p);
requires MoveAssignable<T1> && MoveAssignable<T2> pair& operator=(pair&& p );
template<class U , class V> requires HasAssign<T1, RvalueOf<U>::type> && HasAssign<T2, RvalueOf<V>::type> pair& operator=(pair<U , V>&& p);
It seems that the functionality of (2) is completely covered by (3), therefore (2) should be removed.
Proposed resolution:
In 20.3.3 [pairs] p. 1, class pair and just before p. 19 remove the declaration:
requires MoveAssignable<T1> && MoveAssignable<T2> pair& operator=(pair&& p );
Section: 20.5.2.2 [tuple.cnstr] Status: New Submitter: Daniel Krügler Date: 2008-10-04
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Discussion:
N2770 (and thus now the WP) removed the non-template move-assignment operator from tuple's class definition, but the latter individual member description does still provide this operator. Is this (a) an oversight and can it (b) be solved as part of an editorial process?
Proposed resolution:
Section: 20.5.2.7 [tuple.swap] Status: New Submitter: Daniel Krügler Date: 2008-10-04
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Discussion:
Issue 522 was accepted after tuple had been conceptualized, therefore this step needs to be completed.
Proposed resolution:
In both 20.5.1 [tuple.general]/2 and 20.5.2.8 [tuple.special] change
template <classSwappable... Types> void swap(tuple<Types...>& x, tuple<Types...>& y); template <classSwappable... Types> void swap(tuple<Types...>&& x, tuple<Types...>& y); template <classSwappable... Types? void swap(tuple<Types...>& x, tuple<Types...>&& y);
In 20.5.2 [tuple.tuple], class tuple definition and in 20.5.2.7 [tuple.swap], change
requires Swappable<Types>...void swap(tuple&&);
In 20.5.2.7 [tuple.swap] remove the current requires-clause, which says:
Requires: Each type in Types shall be Swappable
Section: 23.2.3.5 [forwardlist.ops], 23.2.4.4 [list.ops] Status: New Submitter: Daniel Krügler Date: 2008-10-06
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Discussion:
The signatures of forwardlist::remove and list::remove defined in 23.2.3.5 [forwardlist.ops] before 11 + 23.2.4.4 [list.ops] before 15:
requires EqualityComparable<T> void remove(const T& value);
are asymmetric to their predicate variants (which only require Predicate, not EquivalenceRelation) and with the free algorithm remove (which only require HasEqualTo). Also, nothing in the pre-concept WP N2723 implies that EqualityComparable should be the intended requirement.
Proposed resolution:
Replace in 23.2.3.5 [forwardlist.ops] before 11 and in 23.2.4.4 [list.ops] before 15
requiresEqualityComparable<T>HasEqualTo<T, T> void remove(const T& value);
Section: 20.7.15 [func.memfn] Status: New Submitter: Bronek Kozicki Date: 2008-10-06
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Discussion:
Daniel Krügler wrote:
Shouldn't above list be completed for &- and &&-qualified member functions This would cause to add:
template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) &&); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const &&); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &&); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);
yes, absolutely. Thanks for spotting this. Without this change mem_fn cannot be initialized from pointer to ref-qualified member function. I believe semantics of such function pointer is well defined.
Proposed resolution:
Section: 20.4.1 [ratio.ratio] Status: New Submitter: Pablo Halpern Date: 2008-10-07
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Discussion:
The compile-time functions that operate on ratio<N,D> require the cumbersome and error-prone "evaluation" of a type member using a meta-programming style that predates the invention of template aliases. Thus, multiplying three ratios a, b, and c requires the expression:
ratio_multiply<a, ratio_multiply<b, c>::type>::type
The simpler expression:
ratio_multiply<a, ratio_multiply<b, c>>
Could be used by if template aliases were employed in the definitions.
Proposed resolution:
(based on October, 2008 WP, N2798)
In section 20.4.1 [ratio.ratio], modify the ratio template as follows:
template <intmax_t N, intmax_t D = 1>
class ratio {
public:
typedef ratio type;
static const intmax_t num;
static const intmax_t den;
};
In section 20.4.2 [ratio.arithmetic], modify the template ratio_add as follows:
template <class R1, class R2>structusing ratio_add = see below;{typedef see below} type;};Thenested typedeftype ratio_add<R1,R2> shall be a synonym for ratio<T1, T2> where T1 has the value R1::num * R2::den + R2::num * R1::den and T2 has the value R1::den * R2::den.
We would need to make a similar change (struct to using/=) to the synopsis and to each of the other operations described in 20.4.2 [ratio.arithmetic]. Note the addition of type to the ratio template. This typedef allows the previous style to remain valid and permits the use of transitional library implementations for C++03 compilers.
Section: 20.7.12.1.4 [func.bind.place] Status: New Submitter: Sohail Somani Date: 2008-10-11
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Discussion:
With respect to the section 20.7.12.1.4 [func.bind.place]:
TR1 dropped some suggested implementation quantities for the number of placeholders. The purpose of this defect is to put these back for C++0x.
Proposed resolution:
Add 20.7.12.1.4 [func.bind.place]/2:
While the exact number of placeholders (_M) is implementation defined, this number shall be at least 10.
Section: 29 [atomics] Status: New Submitter: Herb Sutter Date: 2008-10-17
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Discussion:
Right now, C++0x doesn't have atomic<float>. We're thinking of adding the words to support it for TR2 (note: that would be slightly post-C++0x). If we need it, we could probably add the words.
Proposed resolutions: Using atomic<FP>::compare_exchange (weak or strong) should be either:
I propose Option 1 for C++0x for expediency. If someone wants to argue for Option 2, they need to say what exactly they want compare_exchange to mean in this case (IIRC, C++0x doesn't even assume IEEE 754).
Proposed resolution:
Section: 29 [atomics] Status: New Submitter: Herb Sutter Date: 2008-10-17
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Discussion:
Right now, the compare_exchange_weak loop should rapidly converge on the padding contents. But compare_exchange_strong will require a bit more compiler work to ignore padding for comparison purposes.
Note that this isn't a problem for structs with no padding, and we do already have one portable way to ensure that there is no padding that covers the key use cases: Have elements be the same type. I suspect that the greatest need is for a structure of two pointers, which has no padding problem. I suspect the second need is a structure of a pointer and some form of an integer. If that integer is intptr_t, there will be no padding.
Related but separable issue: For unused bitfields, or other unused fields for that matter, we should probably say it's the programmer's responsibility to set them to zero or otherwise ensure they'll be ignored by memcmp.
Proposed resolutions: Using atomic<struct-with-padding>::compare_exchange_strong should be either:
I propose Option 1 for C++0x for expediency, though I'm not sure how to say it. I would be happy with Option 2, which I believe would mean that compare_exchange_strong would be implemented to avoid comparing padding bytes, or something equivalent such as always zeroing out padding when loading/storing/comparing. (Either implementation might require compiler support.)
Proposed resolution:
Section: 20.8.13.2.1 [util.smartptr.shared.const] Status: New Submitter: Rodolfo Lima Date: 2008-10-12
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Discussion:
The current working draft (N2798), section 20.8.13.2.1 [util.smartptr.shared.const] declares shared_ptr's constructor that takes a rvalue reference to unique_ptr and auto_ptr as being explicit, affecting several valid smart pointer use cases that would take advantage of this conversion being implicit, for example:
class A;
std::unique_ptr<A> create();
void process(std::shared_ptr<A> obj);
int main()
{
process(create()); // use case #1
std::unique_ptr<A> uobj = create();
process(std::move(uobj)); // use case #2
return 0;
}
If unique_ptr to shared_ptr conversions are explicit, the above lines should be written:
process(std::shared_ptr<A>(create())); // use case #1 process(std::shared_ptr<A>(std::move(uobj))); // use case #2
The extra cast required doesn't seems to give any benefits to the user, nor protects him of any unintended conversions, this being the raison d'etre of explicit constructors.
It seems that this constructor was made explicit to mimic the conversion from auto_ptr in pre-rvalue reference days, which accepts both lvalue and rvalue references. Although this decision was valid back then, C++0x allows the user to express in a clear and non verbose manner when he wants move semantics to be employed, be it implicitly (use case 1) or explicitly (use case 2).
Proposed resolution:
In both 20.8.13.2 [util.smartptr.shared] paragraph 1 and 20.8.13.2.1 [util.smartptr.shared.const] change:
template <class Y>explicitshared_ptr(auto_ptr<Y> &&r); template <class Y, class D>explicitshared_ptr(unique_ptr<Y, D> &&r);
Section: 29.1 [atomics.order] Status: New Submitter: Anthony Williams Date: 2008-10-19
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Discussion:
There was an interesting issue raised over on comp.programming.threads today regarding the following example
// Thread 1: x.store(1, memory_order_relaxed); // SX atomic_thread_fence(memory_order_seq_cst); // F1 y.store(1, memory_order_relaxed); // SY1 atomic_thread_fence(memory_order_seq_cst); // F2 r1 = y.load(memory_order_relaxed); // RY // Thread 2: y.store(0, memory_order_relaxed); // SY2 atomic_thread_fence(memory_order_seq_cst); // F3 r2 = x.load(memory_order_relaxed); // RX
is the outcome r1 == 0 and r2 == 0 possible?
I think the intent is that this is not possible, but I am not sure the wording guarantees that. Here is my analysis:
Since all the fences are SC, there must be a total order between them. F1 must be before F2 in that order since they are in the same thread. Therefore F3 is either before F1, between F1 and F2 or after F2.
If F3 is after F2, then we can apply 29.1 [atomics.order]p5 from N2798:
For atomic operations A and B on an atomic object M, where A modifies M and B takes its value, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B observes either the effects of A or a later modification of M in its modification order.
In this case, A is SX, B is RX, the fence X is F2 and the fence Y is F3, so RX must see 1.
If F3 is before F2, this doesn't apply, but F3 can therefore be before or after F1.
If F3 is after F1, the same logic applies, but this time the fence X is F1. Therefore again, RX must see 1.
Finally we have the case that F3 is before F1 in the SC ordering. There are now no guarantees about RX, and RX can see r2==0.
We can apply 29.1 [atomics.order]p5 again. This time, A is SY2, B is RY, X is F3 and Y is F1. Thus RY must observe the effects of SY2 or a later modification of y in its modification order.
Since SY1 is sequenced before RY, RY must observe the effects of SY1 or a later modification of y in its modification order.
In order to ensure that RY sees (r1==1), we must see that SY1 is later in the modification order of y than SY2.
We're now skating on thin ice. Conceptually, SY2 happens-before F3, F3 is SC-ordered before F1, F1 happens-before SY1, so SY1 is later in the modification order M of y, and RY must see the result of SY1 (r1==1). However, I don't think the words are clear on that.
Proposed resolution:
Add a new paragraph after 29.1 [atomics.order]p5 that says
For atomic operations A and B on an atomic object M, where A and B modify M, if there are memory_order_seq_cst fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modifiction order of M.
Section: 20.8.2.2 [allocator.concepts] Status: New Submitter: Pablo Halpern Date: 2008-10-23
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Discussion:
20.8.2.2 [allocator.concepts] contains a reference to a concept named Dereferenceable. No such concept exists.
Proposed resolution:
Change all uses of the concept Dereferenceable to HasDereference in 20.8.2.2 [allocator.concepts].
Section: 20.5.2.6 [tuple.rel] Status: New Submitter: Joe Gottman Date: 2008-10-28
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Discussion:
In the latest working draft for C++0x, tuple's operator== and operator< are declared as
template<class... TTypes, class... UTypes> requires EqualityComparable<TTypes, UTypes>... bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
and
template<class... TTypes, class... UTypes> requires LessThanComparable<TTypes, UTypes>... bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
But the concepts EqualityComparable and LessThanComparable only take one parameter, not two. Also, even if LessThanComparable could take two parameters, the definition of tuple::operator<() should also require
LessThanComparable<UTypes, TTypes>... // (note the order)
since the algorithm for tuple::operator< is the following (pseudo-code)
for (size_t N = 0; N < sizeof...(TTypes); ++N) {
if (get<N>(t) < get<N>(u) return true;
else if ((get<N>(u) < get<N>(t)) return false;
}
return false;
Similar problems hold for tuples's other comparison operators.
Proposed resolution:
In 20.5.1 [tuple.general] and 20.5.2.6 [tuple.rel] change:
template<class... TTypes, class... UTypes> requiresEqualityComparableHasEqualTo<TTypes, UTypes>... bool operator==(const tuple<TTypes...>&, const tuple<UTypes...>&); template<class... TTypes, class... UTypes> requiresLessThanComparableHasLess<TTypes, UTypes>... && HasLess<UTypes, TTypes>... bool operator<(const tuple<TTypes...>&, const tuple<UTypes...>&); template<class... TTypes, class... UTypes> requiresEqualityComparableHasEqualTo<TTypes, UTypes>... bool operator!=(const tuple<TTypes...>&, const tuple<UTypes...>&); template<class... TTypes, class... UTypes> requiresLessThanComparableHasLess<UTTypes,TUTypes>... && HasLess<UTypes, TTypes>... bool operator>(const tuple<TTypes...>&, const tuple<UTypes...>&); template<class... TTypes, class... UTypes> requiresLessThanComparableHasLess<UTTypes,TUTypes>... && HasLess<UTypes, TTypes>... bool operator<=(const tuple<TTypes...>&, const tuple<UTypes...>&); template<class... TTypes, class... UTypes> requiresLessThanComparableHasLess<TTypes, UTypes>... && HasLess<UTypes, TTypes>... bool operator>=(const tuple<TTypes...>&, const tuple<UTypes...>&);
Section: 30.2.1.2 [thread.thread.constr] Status: New Submitter: Anthony Williams Date: 2008-10-23
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Discussion:
The thread constructor for starting a new thread with a function and arguments is overly constrained by the signature requiring rvalue references for func and args and the CopyConstructible requirements for the elements of args. The use of an rvalue reference for the function restricts the potential use of a plain function name, since the type of the bound parameter will be deduced to be a function reference and decay to pointer-to-function will not happen. This therefore complicates the implementation in order to handle a simple case. Furthermore, the use of rvalue references for args prevents the array to pointer decay. Since arrays are not CopyConstructible or even MoveConstructible, this essentially prevents the passing of arrays as parameters. In particular it prevents the passing of string literals. Consequently a simple case such as
void f(const char*); std::thread t(f,"hello");
is ill-formed since the type of the string literal is const char[6].
By changing the signature to take all parameters by value we can eliminate the CopyConstructible requirement and permit the use of arrays, as the parameter passing semantics will cause the necessary array-to-pointer decay. They will also cause the function name to decay to a pointer to function and allow the implementation to handle functions and function objects identically.
The new signature of the thread constructor for a function and arguments is thus:
template<typename F,typename... Args> thread(F,Args... args);
Since the parameter pack Args can be empty, the single-parameter constructor that takes just a function by value is now redundant.
[ Howard adds: ]
I agree with everything Anthony says in this issue. However I believe we can optimize in such a way as to get the pass-by-value behavior with the pass-by-rvalue-ref performance. The performance difference is that the latter removes a move when passing in an lvalue.
This circumstance is very analogous to make_pair (20.3.3 [pairs]) where we started with passing by const reference, changed to pass by value to get pointer decay, and then changed to pass by rvalue reference, but modified with decay<T> to retain the pass-by-value behavior. If we were to apply the same solution here it would look like:
template <class F> explicit thread(F f);template <class F, class ...Args> thread(F&& f, Args&&... args);-4- Requires: F and each Ti in Args shall be
CopyConstructible if an lvalue and otherwiseMoveConstructible. INVOKE(f, w1, w2, ..., wN) (20.7.2 [func.require]) shall be a valid expression for some values w1, w2, ... , wN, where N == sizeof...(Args).-5- Effects: Constructs an object of type thread
and executes INVOKE(f, t1, t2, ..., tN) in a new thread of execution, where t1, t2, ..., tN are the values in args.... Constructs the following objects in memory which is accessible to a new thread of execution as if:typename decay<F>::type g(std::forward<F>(f)); tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);The new thread of execution executes INVOKE(g, wi...) where the wi... refers to the elements stored in the tuple w. Any return value from g is ignored.
If f terminates with an uncaught exception, std::terminate() shall be called.If the evaluation of INVOKE(g, wi...) terminates with an uncaught exception, std::terminate() shall be called [Note: std::terminate() could be called before entering g. -- end note]. Any exception thrown before the evaluation of INVOKE has started shall be catchable in the calling thread.Text referring to when terminate() is called was contributed by Ganesh.
Proposed resolution:
Modify the class definition of std::thread in 30.2.1 [thread.thread.class] to remove the following signature:
template<class F> explicit thread(F f);template<class F, class ... Args> explicit thread(F&& f, Args&& ... args);
Modify 30.2.1.2 [thread.thread.constr] to replace the constructors prior to paragraph 4 with the single constructor as above. Replace paragraph 4 - 6 with the following:
-4- Requires: F and each Ti in Args shall be
CopyConstructible if an lvalue and otherwiseMoveConstructible. INVOKE(f, w1, w2, ..., wN) (20.7.2 [func.require]) shall be a valid expression for some values w1, w2, ... , wN, where N == sizeof...(Args).-5- Effects: Constructs an object of type thread
and executes INVOKE(f, t1, t2, ..., tN) in a new thread of execution, where t1, t2, ..., tN are the values in args.... Constructs the following objects:typename decay<F>::type g(std::forward<F>(f)); tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);and executes INVOKE(g, wi...) in a new thread of execution. These objects shall be destroyed when the new thread of execution completes. Any return value from g is ignored.
If f terminates with an uncaught exception, std::terminate() shall be called.If the evaluation of INVOKE(g, wi...) terminates with an uncaught exception, std::terminate() shall be called [Note: std::terminate() could be called before entering g. -- end note]. Any exception thrown before the evaluation of INVOKE has started shall be catchable in the calling thread.-6- Synchronization: The invocation of the constructor happens before the invocation of
fg.
Section: 23.2.1 [array] Status: New Submitter: Niels Dekker Date: 2008-11-17
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Discussion:
The Working Draft (N2798) allows access to the elements of std::array by its data() member function:
23.2.1.4 array::data [array.data]
T *data(); const T *data() const;
- Returns: elems.
Unfortunately, the result of std::array::data() cannot be bound to a reference to a built-in array of the type of array::elems. And std::array provides no other way to get a reference to array::elems. This hampers the use of std::array, for example when trying to pass its data to a C style API function:
// Some C style API function. void set_path( char (*)[MAX_PATH] ); std::array<char,MAX_PATH> path; set_path( path.data() ); // error set_path( &(path.data()) ); // error
Another example, trying to pass the array data to an instance of another C++ class:
// Represents a 3-D point in space.
class three_d_point {
public:
explicit three_d_point(const double (&)[3]);
};
const std::array<double,3> coordinates = { 0, 1, 2 };
three_d_point point1( coordinates.data() ); // error.
three_d_point point2( *(coordinates.data()) ); // error.
A user might be tempted to use std::array::elems instead, but doing so isn't recommended, because std::array::elems is "for exposition only". Note that Boost.Array users might already use boost::array::elems, as its documentation doesn't explicitly state that boost::array::elems is for exposition only: http://www.boost.org/doc/libs/1_36_0/doc/html/boost/array.html
I can think of three options to solve this issue:
Lawrence Crowl wrote me that it might be better to leave std::array::elems "for exposition only", to allow alternate representations to allocate the array data dynamically. This might be of interest to the embedded community, having to deal with very limited stack sizes.
The second option, changing the return type of std::array::data(), would break backward compatible to current Boost and TR1 implementations, as well as to the other contiguous container (vector and string) in a very subtle way. For example, the following call to std::swap currently swap two locally declared pointers (data1, data2), for any container type T that has a data() member function. When std::array::data() is changed to return a reference, the std::swap call may swap the container elements instead.
template <typename T>
void func(T& container1, T& container2)
{
// Are data1 and data2 pointers or references?
auto data1 = container1.data();
auto data2 = container2.data();
// Will this swap two local pointers, or all container elements?
std::swap(data1, data2);
}
The following concept is currently satisfied by all contiguous containers, but it no longer is for std::array, when array::data() is changed to return a reference (tested on ConceptGCC Alpha 7):
auto concept ContiguousContainerConcept<typename T>
{
typename value_type = typename T::value_type;
const value_type * T::data() const;
}
Still it's worth considering having std::array::data() return a reference, because it might be the most intuitive option, from a user's point of view. Nicolai Josuttis (who wrote boost::array) mailed me that he very much prefers this option.
Note that for this option, the definition of data() would also need to be revised for zero-sized arrays, as its return type cannot be a reference to a zero-sized built-in array. Regarding zero-sized array, data() could throw an exception. Or there could be a partial specialization of std::array where data() returns T* or gets removed.
Personally I prefer the third option, adding a new member function to std::array, overloaded for const and non-const access, returning a reference to the built-in array, to avoid those compatible issues. I'd propose naming the function std::array::c_array(), which sounds intuitive to me. Note that boost::array already has a c_array() member, returning a pointer, but Nicolai told me that this one is only there for historical reasons. (Otherwise a name like std::array::native_array() or std::array::builtin_array() would also be fine with me.) According to my proposed resolution, a zero-sized std::array does not need to have c_array(), while it is still required to have data() functions.
Proposed resolution:
Add to the template definition of array, 23.2.1 [array]/3:
typedef T c_array_type[N]; c_array_type & c_array(); const c_array_type & c_array() const;
Add the following subsection to 23.2.1 [array], after 23.2.1.4 [array.data]:
23.2.1.5 array::c_array [array.c_array]
c_array_type & c_array(); const c_array_type & c_array() const;Returns: elems.
Add to Zero sized arrays 23.2.1.6 [array.zero]:
4. The presence of c_array_type and c_array() and their semantics are implementation defined, for a zero-sized array.
Section: 20.6.4.3 [meta.unary.prop] Status: New Submitter: Yechezkel Mett Date: 2008-11-04
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Discussion:
The draft (N2798) says in 20.6.4.3 [meta.unary.prop] Table 44:
Table 44 -- Type property queries Template Value template <class T, unsigned I = 0> struct extent; If T is not an array type (8.3.4), or if it has rank less than I, or if I is 0 and T has type "array of unknown bound of U", then 0; otherwise, the size of the I'th dimension of T
Firstly it isn't clear from the wording if I is 0-based or 1-based ("the I'th dimension" sort of implies 1-based). From the following example it is clear that the intent is 0-based, in which case it should say "or if it has rank less than or equal to I".
Sanity check:
The example says assert((extent<int[2], 1>::value) == 0);
Here the rank is 1 and I is 1, but the desired result is 0.
Proposed resolution:
In Table 44 of 20.6.4.3 [meta.unary.prop], third row, column "Value", change the cell content:
Table 44 -- Type property queries Template Value template <class T, unsigned I = 0> struct extent; If T is not an array type (8.3.4), or if it has rank less than or equal to I, or if I is 0 and T has type "array of unknown bound of U", then 0; otherwise, the sizevalue of the I'th dimension of T, where indexing of I is zero-based.
[ Wording supplied by Daniel. ]
Section: 20.8.12.2.1 [unique.ptr.single.ctor] Status: New Submitter: Howard Hinnant Date: 2008-11-26
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Discussion:
20.8.12.2.1 [unique.ptr.single.ctor]/5 no longer requires for D not to be a pointer type. I believe this restriction was accidently removed when we relaxed the completeness reuqirements on T. The restriction needs to be put back in. Otherwise we have a run time failure that could have been caught at compile time:
{
unique_ptr<int, void(*)(void*)> p1(malloc(sizeof(int))); // should not compile
} // p1.~unique_ptr() dereferences a null function pointer
unique_ptr<int, void(*)(void*)> p2(malloc(sizeof(int)), free); // ok
Proposed resolution:
Change 20.8.12.2.1 [unique.ptr.single.ctor]/5:
unique_ptr(pointer p);Requires: D shall not be a pointer type (diagnostic required). D shall be default constructible, and that construction shall not throw an exception.
Section: 20.8.12.2.5 [unique.ptr.single.modifiers] Status: New Submitter: Alisdair Meredith Date: 2008-11-27
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Discussion:
If we are supporting stateful deleters, we need an overload for reset that takes a deleter as well.
void reset( pointer p, deleter_type d);
We probably need two overloads to support move-only deleters, and this sounds uncomfortable like the two constructors I have been ignoring for now...
Proposed resolution:
Section: 20.9.3 [time.duration] Status: New Submitter: Terry Golubiewski Date: 2008-11-30
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Discussion:
duration is missing operator%. This operator is convenient for computing where in a time frame a given duration lies. A motivating example is converting a duration into a "broken-down" time duration such as hours::minutes::seconds:
class ClockTime
{
typedef std::chrono::hours hours;
typedef std::chrono::minutes minutes;
typedef std::chrono::seconds seconds;
public:
hours hours_;
minutes minutes_;
seconds seconds_;
template <class Rep, class Period>
explicit ClockTime(const std::chrono::duration<Rep, Period>& d)
: hours_ (std::chrono::duration_cast<hours> (d)),
minutes_(std::chrono::duration_cast<minutes>(d % hours(1))),
seconds_(std::chrono::duration_cast<seconds>(d % minutes(1)))
{}
};
Proposed resolution:
Add to the synopsis in 20.9 [time]:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s); template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
Add to the synopsis of duration in 20.9.3 [time.duration]:
template <class Rep, class Period = ratio<1>>
class duration {
public:
...
duration& operator%(const rep& rhs);
duration& operator%(const duration& d);
...
};
Add to 20.9.3.3 [time.duration.arithmetic]:
duration& operator%(const rep& rhs);Effects: rep_ %= rhs.
Returns: *this.
duration& operator%(const duration& d);Effects: rep_ %= d.count().
Returns: *this.
Add to 20.9.3.5 [time.duration.nonmember]:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);Requires: Rep2 shall be implicitly convertible to CR(Rep1, Rep2) and Rep2 shall not be an instantiation of duration. Diagnostic required.
Returns: duration<CR, Period>(d) %= s.
template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);Returns: common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type(lhs) %= rhs.
Section: 20.9.5 [time.clock] Status: New Submitter: Beman Dawes Date: 2008-11-24
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Discussion:
Each of the three clocks specified in Clocks 20.9.5 [time.clock] provides the member function:
static time_point now();
The semantics specified by Clock requirements 20.9.1 [time.clock.req] make no mention of error handling. Thus the function may throw bad_alloc or an implementation-defined exception (17.6.5.10 [res.on.exception.handling] paragraph 4).
Some implementations of these functions on POSIX, Windows, and presumably on other operating systems, may fail in ways only detectable at runtime. Some failures on Windows are due to supporting chipset errata and can even occur after successful calls to a clock's now() function.
These functions are used in cases where exceptions are not appropriate or where the specifics of the exception or cause of error need to be available to the user. See N2809, Library Support for hybrid error handling, for more specific discussion of use cases. Thus some change in the interface of now is required.
The proposed resolution has been implemented in the Boost version of the chrono library. No problems were encountered.
Proposed resolution:
Accept the proposed wording of N2809, Library Support for hybrid error handling.
Change Clock requirements 20.9.1 [time.clock.req] as indicated:
-2- In Table 55 C1 and C2 denote clock types. t1 and t2 are values returned by C1::now() where the call returning t1 happens before (1.10) the call returning t2 and both of these calls happen before C1::time_point::max(). ec denotes an object of type error_code (19.4.2.2 [syserr.errcode.overview]).
Table 55 -- Clock requirements Expression Return type Operational semantics ... ... ... C1::now() C1::time_point Returns a time_point object representing the current point in time. Error handling is as specified in ( [semantics.throws]). C1::now(ec) C1::time_point Returns a time_point object representing the current point in time. Error handling is as specified in ( [semantics.throws]).
Change Class system_clock 20.9.5.1 [time.clock.system] as indicated:
static time_point now(error_code& ec=unspecified-name);
Change Class monotonic_clock 20.9.5.2 [time.clock.monotonic] as indicated:
static time_point now(error_code& ec=unspecified-name);
Change Class high_resolution_clock 20.9.5.3 [time.clock.hires] as indicated:
static time_point now(error_code& ec=unspecified-name);
Section: 30.3.1 [thread.mutex.requirements] Status: New Submitter: Pete Becker Date: 2008-12-05
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Discussion:
30.3.1 [thread.mutex.requirements] describes the requirements for a type to be a "Mutex type". A Mutex type can be used as the template argument for the Lock type that's passed to condition_variable_any::wait (although Lock seems like the wrong name here, since Lock is given a different formal meaning in 30.3.3 [thread.lock]) and, although the WD doesn't quite say so, as the template argument for lock_guard and unique_lock.
The requirements for a Mutex type include:
Also, a Mutex type "shall not be copyable nor movable".
The latter requirement seems completely irrelevant, and the three requirements on return types are tighter than they need to be. For example, there's no reason that lock_guard can't be instantiated with a type that's copyable. The rule is, in fact, that lock_guard, etc. won't try to copy objects of that type. That's a constraint on locks, not on mutexes. Similarly, the requirements for void return types are unnecessary; the rule is, in fact, that lock_guard, etc. won't use any returned value. And with the return type of bool, the requirement should be that the return type is convertible to bool.
Proposed resolution:
Section: 29 [atomics] Status: New Submitter: Clark Nelson Date: 2008-12-05
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Discussion:
N2427 specified different requirements for atomic analogs of fundamental integer types (such as atomic_int) and for atomic analogs of <cstdint> typedefs (such as atomic_size_t). Specifically, atomic_int et al. were specified to be distinct classes, whereas atomic_size_t et al. were specified to be typedefs. Unfortunately, in applying N2427 to the WD, that distinction was erased, and the atomic analog of every <cstdint> typedef is required to be a distinct class.
It shouldn't be required that the atomic analog of every <cstdint> typedef be a typedef for some fundamental integer type. After all, <cstdint> is supposed to provide standard names for extended integer types. So there was a problem in N2427, which certainly could have been interpreted to require that. But the status quo in the WD is even worse, because it's unambiguously wrong.
What is needed are words to require the existence of a bunch of type names, without specifying whether they are class names or typedef names.
Proposed resolution:
Section: 20.8.12.1.2 [unique.ptr.dltr.dflt1] Status: New Submitter: Howard Hinnant Date: 2008-12-07
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Discussion:
Consider:
derived* p = new derived[3]; std::default_delete<base[]> d; d(p); // should fail
Currently the marked line is a run time failure. We can make it a compile time failure by "poisoning" op(U*).
Proposed resolution:
Add to 20.8.12.1.2 [unique.ptr.dltr.dflt1]:
namespace std {
template <class T> struct default_delete<T[]> {
void operator()(T*) const;
template <class U> void operator()(U*) const = delete;
};
}